Oh no, not Boss :D

[DiscoFreq]

With help from Sir HC (seller didn't ship outside the US) I could buy a Beckmen B-100 The Boss acoustic preamp.

A very little known pre-Roland Boss product that already has the modern Boss logo. It's in the Boss book and the Boss USA timeline:
http://www.bossus.com/go/boss_classics/

I already had a dual transducer version (with "The Boss" transducers and all papers), but that one is nowhere to find.

[Mr. Maestro]

This is just a guess, but probably some type of contact mic for since it's for acoustics?

[DiscoFreq]

Yes, those are the transducers, they had to be glued in the instrument.

I used them a few times by pressing them on the body and head of my bass (using glue clamps or something like that).

[grygrx]

Interesting find!

[Mr. Min T. Fresh]

Wow!!!

Nice one!!!