The manufacture date would probably give you a good idea. I do not know for certain but as far as I know if there is no designation it is most likely a WT-330. There was a mid Time Traveler WT-390 (which was stamped) and then the current WT-405 (also stamped) both of which were before the WTX's.
The series is identical
AFAIK except for the power and they all go down to 2 ohms...if you are just looking to demo it....the WT-330 would give you a spot on idea of how the WT-405 would sound.
Hope this helps. These are great amps.