Cylinder heads and exhaust ports

Hi. I have three Dommie 99 cylinder heads. They are all marked 22707. One of them is 22707H, one is 22707Y and the third has no letter after the number, it’s just plain old ‘22707’ (there’s no sign of a letter having been removed). All three heads have the same carb manifold fixings -  vertically spaced screws in two pairs.

I decided to use the ‘plain’ 22707 head as it’s in the best condition.  The underside of the head is stamped ‘S.S’ - does that mean it’s a 99SS head? If so, what’s the difference between that and a standard 99 head - it looks identical to the other heads. Or perhaps it’s from an early pre-downdraft 650SS, if such a thing existed?

The ports for the exhaust pipe fixing rings have the same thread pitch but different pitch angles (60 vs 55?) - the ‘Y’ and ‘H’ heads have a more steeply angled thread form than the ‘plain’ head. It’s odd that Norton would change such a basic thing for what seems like no good reason.

The threaded exhaust ports themselves are also a different bore diameter on  the ‘Y’ head which means that the exhaust pipes for that head can’t be used on the ‘plain’ head or the ‘H’ head as the flange that fits into the bore is wider. The difference is only 0.7mm but it’s enough to stop the pipes going in on the ‘plain’ and ‘H’ heads.

It seems odd that Norton would go to such a large amount of trouble to cast  different cylinder heads with only slightly different sizes in some areas for no apparent purpose or gain.

Can anyone shed some information as to what my ‘SS’ head is and why all the heads are different?

I imagine I’ll need to get some ‘one size fits all’ exhaust pipes and carefully grind a bit off the flange diameter where it fits into the head.

Thanks

Regards

Tony