what exactly is Jack Bruce (in '75) playing here?

[Bass V]

1st clue... it's a bass!
after that I've never seen one like it and can't even buy another clue of what it might be.
but now I gotta have one

 

[JimmyThunder]

Dan Armstrong London with a sliding pickup
http://guitarz.blogspot.com/2012/01/1970s-dan-armstrong-london-with-sliding.html
Guitar Blog: 1970s Dan Armstrong London with sliding pickup

 

[ajkula66]

Several well-known names in that "OGWT" performance...Bruce was making some very, very interesting music back then...

 

[Bass V]

Dan Armstrong London with a sliding pickupGuitar Blog: 1970s Dan Armstrong London with sliding pickup

wow, thanx!
but, damn, now I REALLY want one just cos it'll be more unbassobtainium... might be another DIY project.