I know someone who might have some '75 manifolds, if they'll fill your bill. I'll message you his # and you can shoot him a text. Not sure if he's a member here, but he has a Mk III that's converted to a single Mikuni and he gave me the old Amals for the Titanic.
Alan there were 2 types of inlet manifold used on the 850- if you have a RH10 Head the inlet manifold tapers from 32mm at the carb end to 30mm at the head end where as the manifold for the RH4 head is 32mm right through.
I texted my buddy and he was pretty sure he sent them along with the carbs. I checked my bone pile and sure enough, I have 'em. Not to screw up anyone's sale, but as far as I'm concerned, you can have 'em for postage. They were free to me, so I'll pass that forward. PM me your address and I'll get them out if they fit the bill.