Came across some info that says the 7 speed freewheel is actually a bit wider, the 5 and 6 speed are the same size with different size spacers. Dont know about the spacers on the 7 speed.
Supposedly the 6 and 7 speed fit on the same size spiralled spline.

...and problems after the fix i did with swapping the direction of the rear cogs. About two days later, and the morning after I had just rewaxed the chain, I was riding and some clunking started coming from the drivetrain, I stopped the bike and found that pedalling backwards wasnt possible and it appeared to be a link of the chain was seizing up, sometimes they are a bit stiff after a wax. But it kept going, knew it wasnt good, but needed to get to work on time! Closer inspection of what was going on(with my good glasses on) revealed that the freewheel lock ring had come adrift and it was possibly getting caught up in the chain as well as the free wheel floating outwards on the spline......Id tightend it with what was reaonsable force with the tiny bottle opener tool(and believed this things would get tighter as you pedal anyway-wrong)

Bottom line when at home and pulled it all apart, the lock ring was scratched up but still ok, but when putting the freewheel/cassette back on the hub, there was some noticeable play, indicating that the helical splines effectively got grinded out a bit when it was loose. It may have already worn to that stage(but in theory there shouldnt be any wear there if it is firmly attached) and I hadn't noticed it, but given I was expecting some consequences of it working loose it was identified in this case. So rather than put it back on and perhaps it wear more to become very loose I fitted a spare I had purchased for such an occasion. The NOS freewheel fitted snugly, indicating that the wear was limited to the grooves on the freewheel rather than the hub,
So do up that dinky lock ring tight