Christopher (and Bob)…
Original XK120 Inlet Manifold Studs for H6 carburetters without insulator blocks were eight-off C.2386 as fitted up to Engine No. W6918 only. From Engine No W6919 onwards fibre INSULATORS C.7461 were added, necessitating new longer studs C.7488 to be now fitted to all later XK120s and all XK140s up to G5789 (in 1956) when the revised C.11768 stud was introduced, and for spare parts supply superseded the C.7488 stud. Thus if you have the later XK120 SPC published after 1956 all the inlet manifold studs are shown with the post 1956 part numbers, including the eight C.11768 studs for the H6 carburetters with Insulating Blocks in place.
The original C.7488 studs are nominally 4-3/4in Overall Length (comprising a 5/16"UNC threaded end 3/4in long that screws into the aluminium head, then a plain length 3-1/2in long for the gaskets/insulator/H6 area, then a 5/16in UNF thread 1/2in long onto which the spring washer and nut secure the carburetters in place.)
The superseding C11768 are 4-7/16in Overall Length with the difference being solely with the 5/16in UNC length being shorter, now 7/16in long, but still with the same 3-1/2in plain length and same 1/2in long 5/16in UNF length.
The reason for the change from the C.7488 to the C11768 was there were problems with the 3/4in long UNC thread bottoming out in the head casting and breaking into the water jacket, so the head casting was revised to reinforce the inlet manifold stud holes, and the studs were all given a shorter thread so they didn’t bottom out.
So the later C11768 studs can be used now in a pre-1956 head that was originally fitted with C.7488 studs (or indeed the original C.2368 studs when no insulators were used)
You will find all the aftermarket vendors will now only supply the later C11768 studs with the reduced length UNC thread that screws into the cylinder head casting…