Believe you ran into just one of the Bv 20's oddities. Mine doesn't a serial # that I can find either. I have an original catalog & the Bv 20 isn't listed although all the other including discontinued models are. I got a separate sheet with instructions on the Bv 20. I even have an Esman's add where Programmer 200 is $307.86. I bought one after a price increase of $339.95. I still have the box and paper  work it shipped in. Shipping was $3.95.   Sorry  J.W.

No idea. I purchase a BV20 a number off ago the paralux adjuster damaged. I contacted a former Unertal employee who did repair work. He informed me that adjuster is very fragile & damaged easily if stuck. Rather than repair it, I returned it. All the other Unertals have, the serial # is below the Unertal logo. The gentleman I contacted was Aaron Davis, Fairborn, Ohio 45324, phone 937-631-2854. I have no idea if he still exists, considering how old I am.  Good luck!  J.W.