Are you sure about the letter prefix for the serial number? ISMACS doesn't show a "JA" prefix. They do show a "AJ" prefix though.
Visually it looks like a Singer 128.
Yesterday I was given a 1924 Singer. It has knee bar, and takes rocket-ship-like bobbins (vibrating shuttle). It does not go in reverse.
Your machine predates the reverse feature. No problem though, it's easy enough to back tack with it.
It has a motor, but the bearings feel a bit crunchy.
Hunt up some Singer Motor Lube in the tube if you can. The amber/clear/brown stuff, not the white stuff.
Grease the tubes well, but not excessively. You might need to carefully remove the brushes and clean them, their tubes and the commutator. Some brushes are internal and require the motor to be disassembled.
It is missing the bobbin and bobbin case.
Sew-Classic stocks the bobbins, but I'm not sure where to suggest looking for the shuttle. Perhaps your LSMG.
It appears to be missing part of the tension dial.
Your top tension assembly is complete. The earlier ones did not have the numbered dial.
It is missing a belt.
LSMG or Sew-Classic
It is missing the key to the case.
You can use a small flat blade screwdriver to lock and unlock the case till you scare up a key.
Turning by hand, the wheel turns nicely, and while the works feel a bit tired, I am sure that with a cleaning an oiling, that will be fine.
Don't wait to oil it. Get some Tri-Flow oil and oil it now. Then while your working on the green machine the T-F will be doing it's job cleaning and lubing the Singer.
That's a nice little machine. Love those decals.
Joe