Schwinn reset their serial number system (again) in 1965. not sure just when in 1965. The 1965 to 1983 chicago built ef bikes had the first letter for the month, the second for the year, then the numbers. They changed from 5 to 6 digits as production was increasing throughout the 1960s. They reached 1,000,000/year in 1968 and needed more digits.
The number above would be August 1965.
The chicago factory was closed up throughout 1983 in a phased shutdown. They continued the same EF number system on the Murray built Schwinns for a couple years.
Schwinn had many different serial number systems used in the Chicago factory and at all their other factories and oem makers. The Chicago built: Paramounts, the brass fillet brazed models, the Cycle Trucks and tandems each had their own number system. Bikes built in other Schwinn owned plants: Greenville Mississippi, Wisconsin, Washington, Cspel Hungary, and China, all had different numbering systems. Out sourced bikes from Taiwan, Japan and China (and many more unknown suppliers during the Scott ownership) have different number systems by supplier. The Pacific (2002+) era had a few systems including a much longer number system that covers many of the pacific brands. Starting in 2004 a typical pacific number is 5 letters, 2 digits, 1 or 2 letters and 3 to 5 digits. SNFSD04AA12345 for example.
The Schwinn Lookup tool on the web has some errors and only tried to cover Chicago built frames up to 1983.
There have been some slip ups in the Schwinn serial numbers. There was some duplication from 1945 to 1965. One year it took them about 32 days to reset the year. Not a unique event, others have messed up too like the 13th to 15th month Sturmey Archer hubs. There are also gaps when the workers went on strike for months.
How this frame was built is the definitive call on where it was built. The EF bikes are quite distinctive. No one else built bikes using the Schwinn EF process. The Pacific chinese cruisers were welded and would have weld beads at the main joints. Also the weight. An ef frame is much heavier.