Each engine had its own version of the Powerglide - even the two six-cylinders had different transmissions. There were at least five Powerglides in '68, and the L48 version was 3919327; unfortunately, that part number doesn't appear anywhere on the case. The converter was part of the production transmission assembly as-shipped, and unique converters would generate a unique transmission part number.
There may have been a 2-character broadcast code ink-stamped on the case, but I don't have the codes.