They were first introduced in 1935 , there are a few forms. They are used on Blue Star Model J 15-5275 M series Slopers 24-5284 arms will be 15 for Blue Star and 31 for M Series
Y and G Use 33-5042
Empire star Silver Star Gold Star M20 use 66-5135
There are 7/8 and 1” bar sizes.
Early ones have sleeve as part of the top bearing cup, hence why your one don’t fit a M20 yoke.
Where as empire silver gold M20 clamp directly to the yoke
It's never simple! I'll look for a part number under the paint and take it from there. I wonder if a post-war sleeve will fit...?
It can be easy. John has a correct early top yoke for sale for a M20 (see wanted/for sale section). Just buy that 1 and sell the one you have now. It can be difficult to make the correct sleeve and fit it properly, someone in the Netherlands told me as he did this.