PART NO.  SPL.PAGE  DESCRIPTION                                                           
7H7520    15.K.2    Bolt -cylinder to brake plate (1/4") -(4 for front drum brakes)

7H7520 Bolt -cylinder to brake plate (1/4")
MGA front brakes, 1500
MG Midget, A/H Sprite 948cc Front Brakes Triumph TR2, 3, 4 Front Brakes TR2-3 to TS13045 Front Brakes Classic Mini Front Drum Brakes, 1964-84, (10" Wheels) Twin Leading Shoe

Alternate part numbers:
Moss Motors 322-400

That would be a 1/4-28-UNF threaded hex head bolt, but it has a special part number (not standard configuration). Normal hex head for 1/4" bolt would be 7/16" wrench sze, but this one may have a smaller 3/8" hex head.

There is a technical difference between a screw and a bolt. Screw has the thread going all the way to the head. A bolt has some length of unthreaded shank before the head. Usually there is no such thing as a short bolt, but as said, this one has a special part number. Assuming they got the description right, there should be a short length of unthreaded shank just before the head, possibly intended to serve as an alignment pilot diameter in the brake backing plate.