As Paul_WNC said most likely there are no frame markings but you could still look for some stampings on the front frame rails.

I found lots of numbers on the front end.

https://www.tm9-801.com/forums/viewtopic.php?f=9&t=1335&p=2932&hilit=stampings#p2932

Also some have been luck enough to get whole SN's and model info:

http://www.surfacezero.com/g503/data/500/medium/Frame_Serial_Number.JPG

So at least look... esp on a small run model like yours... most likely sent off to a sub-contractor for modification.

Last edited by pfarber; 08/01/2015 4:07 PM.