As I was disassembling my Hercules Royal Prince to give it a deep cleaning, I noticed that the left BB cup has a little "56" stamped on the outer edge. I've been trying to arrive at a date for this bicycle and this is the only clue I've yet to find on this machine since both wheels had been replaced.
Is the "56" stamped into the cup a date code? See pic below...
Brampton BB Cup by
Sallad Rialb, on Flickr