My '57 Spit has a screw-on cap, and I don't see in any of the subsequent BSA Spitfire sales lit here that there was a butterfly cap.
The '62 "Gold Star Twin" had a "quick action racing filler cap with vent", per BSA's US sales lit.
Darn shame that the Spitfire didn't have the UK sales promotion that we did here in America.
David