Hi studio,
Yes, the 402 is always the 6.5" deep model, so I suspect the "400" is a typo on that seller's part. Some people erroneously think "400" refers to all Supraphonics.
From the examples I've seen, the only stamp Ludwig ever gave on Supras is a date stamp on the inside of the drum. My '66 Supra has a month/year stamp, and my '99 has only a year stamp. (Both 400 models)
Generally speaking, the 402 details are 6.5" x 14", aluminum "Ludalloy" shell, 10 imperial lugs, triple-flange hoops, P83 or P85 throwoff (depending on era), raised center bead. The badge will vary according to era, as mentioned below.
If there is a lot of pitting, it's probably the aluminum model, since chrome plating mated with brass much more successfully.
Without access to the drum, I'm not sure how you could confirm if it's a brass shell or not. If you had the drum, you could most likely tell by the weight and tone. You could also remove one of the lugs, for example, and look at the shell's cross-section to see if it's brass.
The ol' magnet test is not reliable with Supras, since magnets do not attract well to either brass or aluminum.
Hope this helps,
Charles