I just picked up a "Golden Jet" today, never heard of it and can't seem to find anything about it. It's sporting this serial number and an Excel Mark II Hub. I've rebuilt a few Mark II hubs, so that will not be a problem.
Patina looks good from the dropout. My AMF number chart only goes to 1963 with an N number, but if they kept up the lettering sequence per year, "U" would be 1970, unless they skipped the O, making it a 1969 bike, or 68 if they skipped Q also.