750 rear brakeplate to sprocket clearance

The clearance between the brakeplate and sprocket/drum was such that the lip of the plate did not enter the annular groove on the edge of the sprocket. The rear sprocket dates back to early 1980s and I assumed it was a poor pattern part. A recently purchased and reputable sprocket exhibits exactly the same issue, the gap is sufficient that the brake shoes do not fully enter the drum and have an unworn lip at the edge. I have checked the assembly from all the exploded diagrams I can find and all is in order. An internet search only shows people with the opposite problem, ie. circlip failure and clearance becoming too small. Other than having an inaccurately made stub axle I am at a loss and would welcome input

Cheers Pete