Well, 37 is Amex, I believe that 373732 does exist (quick Google), and all Amex card numbers are grouped 4-6-5 digits. So that’s easy.
For the chargecards, it was always the case that digit 12 was the issue number, 13 and 14 were the number of the card on the account, and 15 was the check digit.
I worked out 12, 13 and 14 by having multiple users on a chargecard and having a few of my own lost/stolen/compromised. My first platinum card was 100 in digits 12 to 14. After it was lost or stolen the next one was 200 (and the check digit changed of course, standard Luhn algorithm to calculate that). My wife’s Supplementary card digits 13 and 14 were 01 and my Additional Card was 02. My parents had supplementary cards too and those were e 03 and 04.
I’m a very very sad individual 
Edit. Same thing doesn’t, as far as I know, apply to all Amex cards including some newer credit cards - but I’m not certain as I don’t have one.