there's a screw when you tilt the machine back - might be right in the middle somewhere (it's by the bobbin case on the 301s) but on the 319 and possibly the 15 it's apprx in the middle of the bottom - just a big screw turns to the left to lower, tightens to raise back up....
I'm not sure if all 15s have the lowering feeds
but here's the book:
parts.singerco.com/IPinstManuals/15.pdf
if the price is reasonable - (cheap) grab the machine no matter what LOL