Dean, as someone already suggested, the bolt may be SAE.
I checked a Raleigh Pro MkV which has a bolt threaded into tapped SP clamp as you described. The bolt is a 6mm Allen-head cap-screw that has a total length of 1.25" of which 1.00" is threaded portion. My cheapie thread-gage indicated perfect marriage with a gage marked "1.25". I don't know whether this gage is metric or SAE but was with my SAE Tap&Die. I also can't verify whether my bolt is original or not...it does use a thin flat-washer with same diameter as Allen-head. Bolt appears to be SS, although it is magnetic, and there is a "T" embossed/forged inside the Allen recess.