Maybe it's time to mate up our lists again BP!
Just looked and it seems that I have a F 76xx as a true Mk1A. So there is a cross-over/criss contamination at the final changeover period. This isn't totally unusual as it seems to have occurred at a other points too. I put it down to multiple production lines, rectfication etc etc.
We need to ask SBK this question. Does your gun have longitudinally fluted ribs around the gas cylinder OR does it have a simple slab sided gas cylinder