I think that the body is from a different pistol to the slide. Worth measuring the body to ascertain whether it has been cleaned/linished to eliminate a previous serial number. After all, the BODY is the master component and certainly in the UK, was never available as a spare part
I wouldn't necessarily agree that the SU serial number would be prefixed by O and there are plenty of precedents for this. When you are buying, you dictate! When we were buying L9's, we told them what format we wanted. Same as L7's et al......