The M20/M21 big end bearing is the same for all years from 1937 to 1962. It was also used on the BSA B series models except for the Gold Star I think. It comes as a complete assembly of pin, cage, rollers and outer sleeve. After fitting the outer sleeve to the rod it will need to be honed which is something most owners can't do and will need specialist help.
Any shop selling BSA parts should be able to help. And again, it does not matter what year your M20 is or what numbers are cast in to the flywheels. They all use the same big end.
I do believe Mark Cook on this forum can also supply you with a big end.
Occasionally a big end does turn up with 'crowded rollers' and no cage...They may have been after market items back in the day or perhaps a BSA big end with the cage discarded and more rollers fitted as the cages can get damaged/worn...I've never checked to see if that arrangement is dimensionally possible.. However, they are uncommon...In 50 years and over scores of different engines I've come across only four or five of the type...Ian
