My Google-fu hasn't been particularly successful - but it seems you can get a hold of a service manual ... for a fee.
Without that, I'd be looking for some other markings. A photo could help as there are a few people here that have seen enough components to be able to offer some useful comments if they could actually see the pot.
With the wiper having fallen off, you might be unable to determine the resistance profile (log, linear, anti-log, etc.) but you should be able to identify its total resistance. Just measure the resistance between the two outer lugs (this will be more accurate if out of circuit).
With a loose wiper floating around, it might be an idea to tip the pot around in different orientations and give it a shake so as to make sure the wiper isn't shorting anything internally and take some more measurements to confirm. As a final step, I'd suggest cutting the connection between the two joined lugs to minimise the possibility of something shorting inside that will affect the measurement.
Once you have this measurement, get a hold of a couple of (garden variety) pots with this resistance (or slightly more) in linear and log (these are the most common tapers) and try each of them in turn to see which functions as closely as the original. Armed with the resistance and taper, you can then focus on getting a mechanical match ... with the quality of the movement you need.