Dutch - can’t see why it confused you, it was just straightforward detective work.
Someone lists a part number. I look it up in my AE catalogue and it doesn’t exist. Then Musky produces a BSA chart listing that number. Ironhead (not Bob) then suggests it’s for an M21. So I look up BSA M21 in the catalogue, cross-reference the dimensions and give the AE number listed. That’s how it was ‘derived’. Maybe the original number was superceded? (my earliest catalogue is ’73). Was only trying to be helpful, but it seems some don’t appreciate that.