Yup, GM sold the TH400 to several outside companies.
Yes, the 534 is a modified Julian date - the 169th day, aka June 17.
The Gibb CY tags are atypical in their stamping, as you mention. The first part of the code is normally pre-printed on the TH400 tag, but these trans used prototype trans tags. The tags are generic so the preprinted part is missing, only the Julian date is stamped.
Maybe that is what caused the issue with the POPs - the clerk didn't know how to enter it on the POP since it wasn't on the tag in the normal format.
http://www.camaros.org/drivetrain.shtml#TH400