Macybaby

the letter that is part of the model number designates the plant

The letters preceding the model number sometimes help you determine where it was made because Singer typically did that. But it's not a "code", Singer just started at A at one plant, B at another and so forth. Then if they ran out or wanted to change, Signer moved to another letter, or added a second letter for those plants. You'll see E, EA, EB, up to EX, though they seem to have skipped letters that would be hard to tell apart on the stamp. I think they went to FA after EX (they'd already used F in the early 1900's)

The E and F series appear to be numbers used in the Scotland plant.

And for fun - Singer used G

in 1910 they started at G000000 to G999999 (six digit numbers)

less than a year later, they needed more numbers so used
G1000000 to G9999999 (seven digit numbers)

That took them up to about 1924, and, still needing more the went back and used up

G0000000 to G0999999 (the seven digit numbers you get by adding another 0 to the front of the six digit numbers)

And that got them another year, then they had to use a different preceding letter.

This is the most common area people miss identify their machine - they don't take into account the changes from 6 to 7 digits.