PNT
This is the PAINT CODE.  For more detail, and for color samples, see my web page on DeSoto COLORS.  The first letter is the roof color.  The second letter is the main body color, and the third letter is the lower body sweep, if any.  Here are the codes for the colors:
    A    Black, all models
    B    Capri Blue, Fireflite and Firedome
    C    Azure Blue Metallic, Fireflite and Firedome
    D    Seafoam Green, Fireflite and Firedome
    E    Tamarack Green Metallic, Fireflite and Firedome
    F    Dove Gray, Fireflite and Firedome
    G    Charcoal Gray Metallic, Fireflite and Firedome
    H    Fiesta Red, all non-Adventurer models
    J    Muscatel Maroon Metallic, non-Adventurer models
    K    Sunlit Yellow, Fireflite and Firedome
    L    Surf White, Adventurer
    L    Frost White, Fireflite and Firedome
    P    Adventurer Gold, Adventurer
    R    Mandarin Rust, Fireflite and Firedome
    V    Spanish Gold, Fireflite and Firedome, Spring Special
    X    Sahara Tan, Fireflite and Firedome
    Z    Spice Brown Metallic, Fireflite and Firedome

Thus, my Firedome sedan, coded XRX, has a Sahara Tan Roof, a Mandarin Rust body, and a Sahara Tan lower body sweep.

TRM
This is the combination of material types and colors for the car interior:

    These are the Firedome 4-dr sedan codes:
    501    Blue cloth/vinyl
    502    Green cloth/vinyl
    503    Tan cloth/vinyl
    573    Gray cloth/vinyl

    These are the Firedome Convertible & Sportsman codes:
    511    Blue "Silver Brocade" cloth/vinyl
    512    Green "Silver Brocade" cloth/vinyl
    513    Beige/gray cloth/vinyl
    517    Red/gray cloth/vinyl
    611    Blue vinyl
    612    Green vinyl
    613    Beige/gray vinyl
    614    Red/gray vinyl

    These are the Fireflite 4-dr sedan codes:
    531    Blue cloth/vinyl
    532    Green cloth/vinyl
    533    Gray cloth/vinyl
    534    Gray/coral cloth/vinyl

    These are the Fireflite Convertible & Sportsman codes:
    541    Charcoal gray "Westminster Tweed" w/blue vinyl
    542    Charcoal gray "Westminster Tweed" w/green vinyl
    543    Charcoal gray "Westminster Tweed" w/white vinyl
    544    Charcoal gray "Westminster Tweed" w/copper vinyl
    545    Charcoal gray "Westminster Tweed" w/red vinyl
    641    Blue vinyl
    642    Green vinyl
    643    Gray/white vinyl
    644    Copper vinyl
    645    Gray/red vinyl

    These are the Fireflite Station Wagon and the special Adventurer codes:
    546    special Adventurer gold trim (Adventurers only)
    623    Tan vinyl (station wagon only)

Here are samples of the available cloth, and the vinyl. Click to enlarge.

TR
This stands for TRANSMISSION. Fireflites and Adventurers only came with Torqueflite trannies, so they have nothing stamped there. Firedomes can either have "nothing" which indicates the MANUAL transmission, or a "3" indicating the Torqueflite 3-speed (a $203.75 option).

AG
This stands for ACCESSORY GROUP.  1957 DeSotos had several available packages.

GG
This stands for GLAMOUR GROUP.  There was only one available 1957 DeSoto package, but it varied depending on which model it was installed on.

    337    Glamour Group #7  For all 4-dr sedans and
           wagons
           Front bumper guards, roof molding package #1,
           stone deflectors, and wheel covers

    337    Glamour Group #7  For Firedome and Firesweep 2
           and 4-door Sportsman only
           Front bumper guards, roof molding package #2,
           stone deflectors, and wheel covers

PS
This stands for POWER SEATS. The options are:

4 - Your car came with power seats

nothing - Your car does not have power seats

PB
This stands for POWER BRAKES. The options are:

9 - Your car came with power brakes

nothing - Your car does not have power brakes, or is an Adventurer which came standard with power brakes

AH
This stands for the AIR CONDITIONING/HEATER group. The possibilities are:

1 - Your car came with A/C and fresh air heater

4 - Your car has a fresh air heater

6 - Your car came with the gas-fired instantaneous heater

nothing - Your car either has no heater, or you have an AG of 1, 2, or 3 (Accessory Groups include the heater)

PW
This stands for POWER WINDOWS. Your possible numbers are:

5 - Your car came with power windows.

nothing - Your car does not have power windows.

RM
This stands for ROOF MOLDING. The possibilities are:

1 - This is the stainless steel roof package for 4-dr sedans and station wagons

2 - This is the stainless steel roof package for Firedome 2 and 4-dr Sportsman models

nothing - Your car either does not have the roof molding package, is a Fireflite Sportsman or Adventurer which came standard with the package, or you have GG 7 (Glamour Group which contains a roof molding package)

SS
This stands for STONE SHIELDS. These are the curved stainless pieces at the leading edge of the rear wheel opening. These were an accessory for all models at $6.26 extra cost. Your possible numbers are:

9 - Your car came with the shields.

nothing - Your car is correct without them. OR, you have Glamour Group code 7, which includes stone shields.

UC
This stands for UNDERCOATING. This was an option on all models at a cost of $13.20. Your possible numbers are:

1 - Your car came with undercoating.

nothing - Your car is correct without it.

SPL
It's reasonable to assume this means SPECIAL, as if ordered with something unusual about it, or for export. For example I know of one car with this item coded 1371. That car has a paint code of 001, not found in any 1957 DeSoto reference. My assumption is that this was a special order non-standard paint color.

CS
This stands for COLOR SWEEP. The possible numbers are:

1 - Your car came with the stainless molding piece only, and the lower body color matches the main body color.

2 - Your car came with the stainless molding mentioned above, PLUS a different color lower body section, the "sweep". This was a $26.35 option

nothing - Your car EITHER is a Fireflite convertible, Fireflite 2 or 4-dr HT, or an Adventurer (all of which came STANDARD with the color sweep), OR your car has neither the color sweep nor the molding.

BG
This stands for BUMPER GUARDS. These are the bullet-shaped pair of front bumper extensions and the wide bands at the rear edge of the rear quarter panels, extending down along the side of the car from the bumpers. This guard package was a $16.80 accessory for all models. Your possible numbers are:

3 - Your car came with the bumper guards.

nothing - Your car is correct without them. OR, you have Glamour Group code 7, which includes FRONT bumper guards.

The remainder of the codes I haven't cracked yet.

IF ANY OF YOU KNOW THEIR MEANING, PLEASE EMAIL ME!!!  Again, If you own a 1957 Desoto and can send me your car information and the cowl tag codes, I'll try to piece this all together!