crank to barrel fixing studs

i have 9 studs which fit into the cases to fit the barrles onto. 2 are stepped to a smaller size 6 are all the same and the 9th is the same diameter as the 6 studs but longer. looking at the diagram i have the 2 stepped studs fit on either side at the front, the 6 the same fit around the back, but the single hole in the middle at the front is far shallower than all the others and the longer stud i have left is not an obvious fit, diameter is fine but it only goes in half its threaded lengh. what have i done wrong this time?

Somewhere around 1972 Norton changed from 3/8-16/26 studs to 3/8-16/24. Prior to 1972 there should be the two stepped studs and 6 studs 1 7/8" long and 1 stud 1 1/2 " long (front). Then in '72 these were 6 studs 2" inch and the front 1 1/2" (24 tpi). In the '73 parts book the 750 shows all 7 as 2".

It's possilbe you have a mix of early and late studs. Are they 26 tpi at the nut end or 24 tpi? Is it a pre-20XXXX engine number?