What a can of worms!
The Byron Figures appear to utilise evidence from a wider base ( including the USA) than the RYA, and show the 275 off 1042 with a two-bladed fixed prop, and the SS 26 off 987 with the same prop configuration.
I have always said that the number for the SS26/27/275 was likely to be harsh (particularly on the 275/285) because:-
I believe the 986 figure applied to the first outboard powered boats; and
enormous weight has been added over the years, as the fitout has improved.
Personally I treat the the published displacement figures for the 275 with very considerable scepticism. I would reckon nearer 6000lbs myself.
If I recall corrctly Kate, at Topsham you have the additional complicating factor of the SS 26's having increased their "P" measurement by dropping their booms a foot or so? That renders the use of the "official" 986 dubious in the extreme, certainly without a compensating allowance to other boats racing in "standard" trim for their respective classes.
I gave up PY racing years ago because of the total lack of consistency apparent in the system, and my perceptions of local bias (why are the local rating "bandits" always well-connnected to the local handicapping mafia ?). I went off and measured my previous boat for IRC, and played with the big boys. The "rule" is still imperfect, but at least you know it is reasonably scientific, and that given your boats conditions and a good race from the crew, you have a modest chance of success!
I do wonder if there are any SS 26 and 275 out there measured with Endorsed IRC ratings so that there is independent evidence of displacement?
I suppose a call to Seahorse would find out.
275/30 "Cascada"