Hi tromper,

I have seen several bikes from the 1960s and early 1970s where people have scratched numbers into the frame or handle bars or where ever. I think this was done in order to identify the bike after theft, therefore anti-theft if potential thieves see it. It probably worked much better in those days then it would today. Many of today's thieves are organized and move stolen bikes thousands of miles to be resold or grind down identifying marks. Also, as you say, it may have been done by a business who rented bikes, also as a means of identification if stolen or lost.

I am interested to know if the head badge on your "Super 5" says "NISHIKI" or "American Eagle", or if there is an "American Eagle" decal at the bottom of the seat tube.