The wheels themselves (both three and four spoke) are universal items - only the bosses are different. They have the same 8 hole bolt on boss style that you see on Momo wheels etc...

From what I remember, 4 spoke wheels were standard on RS1600i's and came with a non horn push boss. They also had "nobbles" on the back of the wheel at 3 and 9 o'clock (for grip)

The three spoke rims DID come in two different thcknesses, and were options from the Ford RS Options catalogue. These were availabel for the S1 and the 3dr Sierra (bosses), and later when the Saff Cos came along and the S2, Ford carried on the wheels and updated the bosses accordingly (for hornpush bosses)

Bosses were also available for mk3 Fiesta's swell - other marques aswell possibly, but I only know of the mk3+4 Escort, mk1+2 Sierra and Mk3 Fiesta for 100% definite (I have had a boss for a Mk3 Escort (4spoke wheel), bought a 3spoke that came fitted with a boss for a mk3 Fiesta, and my mate had a 4spoke on a S2 with hornpush boss, and have seen pics of both Mk1 and Mk2 Sierra's wearing them with Ford bosses)