I'm guessing the # is production run maybe?
These days Lincoln Welders have parameters set. If you move to slow for the wire speed and amperage setting, it alarms. They even make helmets with green and red lights. Green = proper work angle. Haven't seen these in production yet but in training. A lot of Fanuc robotic weld cells that don't miss. All monitored by camera. Our bikes were made in the very primitive years of automated welding. Simple turn fixtures with phuematic clamps. Guessing this because of old equipment I've worked on. Thin wall tubing with trick engineered gussets.My guess is it's the serial number of the jig the frame was built up in. Any frame QC problems would need to be traced back to the jig it was built in.
Wasn't it after WW2 we went and help to rebuild Japan? Helped in the setup of their manufacturing processes as well? Few decades later they came over here and passed us on work ethic, quality, and wages a lot of times. I was not alive. Just know what Ive read and heard.