If it is a VW case, it should be stamped on the side of the sump, the side the thermostat is on. On a Spyder, this is the driver's side.
It is VERY common to not have a serial number stamped below the generator stand, replacement cases don't get stamped. Mine has a Jake Raby(RAT) and a number.
An AH trans case is perfect for a Spyder: it's a 1968 to 1972 dual side cover case, just before VW switched to the single side cover case for rear-wheel drive only.