I believe there might be some confusion on what is a Cromwell 6-pdr and what is a Cromwell 75mm, as 6-pdr guns were converted to fire US 75mm ammunition. The 75mm was problematic, which is why the 17-pdr was designed, but also why the British continued to use the 6-pdr after the war. I suspect the numbers of "75mm" could be misleading, if the MOD was recording ammunition rather than guns. I could be completely confused in the first place also, but this is my understanding of it.