RE: K&B .61 engine?
Tim:
Your engine is either a 6330 or a 6550. It does have the "Block" carb, supplied in production between the ones that came with the Perry carb, and the current ones with the "Ribbed" carb.
The only way to tell the 63xx from the 65xx is the prop drive washer. The 63xx had a flat in the crank mating with a cast drive washer, the 65xx uses a plain round crank, a centering cone, and a fully machined drive washer.
There's no great amount of "Stuffing' of the case betwen the early and late production, but the early engines were slightly smaller inside. The late backplate wont fit the early case.
Bill.