Series 2, Mine has the 23 DGA head installed, bought used. She gave me a lot of trays,tongs,etc and in the drawer is a knob.
Looking over the manual it appears to be the Condenser stage adj. Knob. That shaft is there but just has a screw
in it so u can't adjust for 35mm,120 etc. Its stuck btwn 35 and 2 1/4 by 2 1/4 and even with visegrips i can't
turn the shaft. Weird thing is with the color head on, theres no room for that knob (it would hit the head).
So with the color head you don't need to adjust?
That doesn't make sense to me.
I did some 120 film last nite and was getting vignetting but then switched from the 50mm lens to 105 that solved that.
(still waiting for manual so working things out)

2- Theres a BIG unmarked lens also in the junk drawer she gave me, condenser lens but have no idea what its for.