I would guess relatively early too, because of decals and the black color. I'm not sure, there were overlap in productions regarding all the features; 15s with screw-nut stitch length adjuster and no reverse are known to have serial number as late as 1950. Around 1954-55 they can turn up in beige paint and with an updated look, and even pale green. I don't know if model 15 ever was cast in aluminium, but around this time the 201K was. In the US it looks like the 201 continued as a black cast iron machine. Your machine would probably have a manual listing it as a model 15J-88.