That is a coincidence, I bought one from a bootsale on Sunday as well.
It appears to be designed to be part of a Wheatstone bridge circuit.
It is nicely made and all works pretty well but seems to have a permanent 20 ohm minimum resistance, I have not found out why yet.
Has anyone got any information on this make and model, age, its use, or why it has a 20 ohm bias. ?
I bought it as I was interested to see what was in them and how they were made, not sure what I will do with it, I will probably sell it on.