the rear manifold on mine has been welded, feasibly on a differing angle. the front is a standard ET32/16. do the pair of "D" manifolds work front and rear on B/C layout heads or has someone modified my rear one for some alternate purpose? i ask as i recently saw the same setup also cut and shut on a local "B". i have no part number left on the rear to determine it having been a rear or a second front "D" part.
Good point on the D manifolds. Of course the rear one wouldn't fit on an earlier motor because Ds had two front heads. Doh! Dunce cap time! Hence the welding. The front one would be fine but, yes, you'd have to alter a rear manifold to fit a B or C rear head. Before you do, though, it's worth asking around because older Vincent owners with boxes of old parts sometimes have things like this lying fallow...
Clip fitting Monoblock adapters were available to put flange mounted carbs onto standard carb stubs. Mk1 Concentrics will fit onto these. Could these be Z276/6/201 Carburettor Flange Clip 276 available from the club shop? A phone call to them should confirm.
Last year the VOC Spares Co supplied me with an aluminium manifold, ET32/20. It has a flange at each end allowing me to put an Amal Monobloc on the rear cylinder of my "B" Rapide. Presumably a Mk1 concentric will fit on the same manifold.