Forging number with a 68 prefix makes me think it started life on a later unit construction A50/A65. 68 7011 is the forging number you can see, 68 7010 is the part number for a pre 1966 Unit Model brake pedal. Here BSA are still retaining the familiar single digit difference between a part number and the raw forging or casting. Cheated a bit here, checked it out on Drags' Website.
With the forging number so close to the inner edge, possible chance a piece has been chopped off, and as a guestimate this is the reason for the 14mm overhang. So a spacer here to support the coils of the spring may save the day and will positively locate the cross shaft until the right part comes along.
So it does fit the splines, but not quite the right part for the job as it is. Basic spring layout follows Col's, coils go around the pedal splined boss or in this case the added spacer if my assumption proves correct. On my frame the splined shaft extends 35mm from the S/A tube face plate, so the pedal boss plus any spacer needs to be at least 35mm.
Swarfy