The only rule I know of is that we don't know why they seem to represent almost every year and maker, in particular early Savages. I had one that was a pristine 1944 N322-- M47C. Any good reason for that not to be converted? None that I could see. My best guess would be they got ahead of themselves at some points, perhaps when the supply of brackets fell off due to ?? and the extra were set aside and never caught up with again. It's been said that things like solid forsight blocks led to rifles being rejected, but why would you convert a rifle then reject it when you could see very easily at the beginning what type of foresight block it had?
Bear in mind that we figured out roughly what H&H produced and it wasn't a lot before 1944. The Canadians didn't get their full stock of No.4(T)s until around early 1944 IIRC. The supply of telescopes far, far outstripped the conversion work based on serial numbers and dates of scopes.