There are two easy ways to check:
HS4's are 1 1/2 SU's which mean that the internal diameter of the throttle body is 1 1/2" where it is bolted to the inlet manifold, compared to HS2's that are only 1 1/4" in diameter, and HS6's are 1 3/4" in diameter.
HS4's are bolted to the inlet manifold by two studs that are offset at 45 degrees to the vertical when looking at the inlet manifold side, i.e. they use the two corner studs of a square stud arrangement around the mounting flange. HS2's also use two but they are vertically aligned on the manifold mounting flange, and HS6's use four studs. Thus, it is possible to put HS6's (1 3/4" dia) onto a manifold designed for HS4's (1 1/2" dia) (with some manifold modification) as the mounting holes will line up, but not HS4's onto a manifold designed for HS2's.
There are a few exceptions to this with very early SU's of the 1950's.
I've just come across a useful set of books dealing with our beasties breathing gear, namely the SU workshop manual and SU reference catalogue, both available from Burlen Fuel Systems, 01722 412500 or email email@example.com, they are charging 7.95 for the two books at present.
Tony Childs gave me a url (http://www.users.globalnet.co.uk/~alecks/) for a place the sells the exact bracket, bell crank, and linkage bar w/clips needed for conversion. The place is called Spit Spares. They got back to me and will sell the kit for $15. Now, we all know, ya can't buy ANYTHING for Spitfires for $15, so this is just great !! BTW, Spit Spares also sells the carbs and manifolds, and I'm sure lots of other stuff.
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