Thanks all for the contributions, I needed to pick up my meter and I needed a moment from crisis for hobby stuff. Turns out the negative probe on my meter has a hard plastic sheath around the tip, leaving just a pinpoint. Fine for detail work, not so much for tight spaces. It does not fit into the outside shoe slots, to reach the metal contact there.
I got some readings using the positive probe in the pc hole, and tripping the flash. The range of 13.9v to 4.9v that i got fits with the 14.1 shown on the dpanswers website voltage list for the Vivitar 2500 i was measuring.
Unfortunately, by the time i (later) put my mystery adapter on the same 2500, my meter had stopped reading anything. The display shows strong visible zeros, but never budges from that. Maybe I blew out a probe?
Whatever, I will need to have a functioning tester - this one says every battery in the house is dead... ;-)
I'll get back to you.
Even if these things prove to be voltage reducers, it doesn't answer how they came to be. If home or shop made, the assembly quality is high.
I'll try taking one apart.
Thanks again/
P.S. I read that reverse polarity is not at all good for digital slrs - not sure if it makes a difference in the mechanical cameras, or old TTL systems.