ID this bass please....

Not at all sure, just guessing Dan Armstrong or Ampeg?

 

My guess is an Epiphone ET-280 without the metal covers.
Epiphone bass guitars

 

Ampeg/Armstrong

And why does Jack's head look so big in that photo?

 

Bridge, pickup and headstock don't match Ampeg/Armstrong, and I don't think that body is Plexiglass. Maybe a prototype?

 

Jack Bruce bass ?

eidt: i'm gonna guess a wooden dan armstrong/london: 1972 Dan Armstrong London

edit again: okay, looky here: Dan Armstrong - The Man and his Guitars

 

Armstrong didn't make only clear plexiglass instruments. He even dabbled in reworked Danos.

Here it is, wood body, sliding pickup and all:

Dan Armstrong - The Man and his Guitars

LOL, looks like yodedude2 beat me to it.

 

let's call it a tie