Well, I dug up more info. Schwinn had another plant in the 80's they started up when unions were a problem in Chicago, in Greenville, Mississippi. It was in use into the early 90's. So your bike may be American made.
"During the 1980s, Greenville used the old letter-digit
(e.g. F8 = June 1988)system which Chicago abandoned in 1965,
when it changed to the two-letter system."
As for the Y serial number, I found this:
"Some Japanese bikes may have an 'X' in the month position of the frame serial number. This was used to connect the remainder of a production run that spilled over into January of the next year. For example ‘X’ may have represented an overrun from the previous year. X9201937 should be Month X and Year 9. But it might be Month January of Year 0. Using this logic, Y9201937 might actually have been made in February of 1980, but was a part of the 1979 production run. I'm still looking for a confirmation of this dating method."
So, I'm thinking it is most probably from a run that spilled over in Greenville and they used that type of numbering like the Japanese bikes did.
The Japanese bikes were imported 71-82, made by Panasonic, before the Taiwanese made by Giant bikes were imported after 81. The Giant frames serials start with a "G".
But the Japanese and Taiwanese frames were mostly road bike frames, that's why I think yours is a Greenville made frame.