If the transmission is really from a GN/T-type if will have a tag near the tailshaft which will have the code BRF. If the paint on the tag is unreadable, BRF will be visible stamped into the tag itself along with lots of other numbers.
However, even with this tag, it is still a good idea to remove the pan and have a look at the valvebody to verify that it is the original one. Without the original valvebody, servo, and governor, this 2004r is the same as any other regular 2004r found in your average grocery getter. The valve body should have a splash of pink paint on it as well as a paint stamp, usually in white, with a marking of BR#, where # can be a 1 or a 2 etc. This will help confirm that the transmission is an original.
If the torque convertor is original it will have a bunch of the number 5 stamped around its edge. If there are 7's then it is just a regular convertor and not the higher stall GN convertor.
If it really is a GN trans, then $100 is a bargain!
