I have a 1951 15-91. I had the needle in backwards and was threading it wrong. The flat side of the needle goes toward the left side and the needle threads from right to left, like the featherweight. Those are the only 2 machines I have that thread that way, but they won't sew a stitch unless they are situated just so!

Add Singer 293 and Singer 301 to your list. Gave me fits until I figured that out. The throat plate has an arrow - should have been a clue. The arrow points the way the thread goes in the needle. If the thread goes in that way the needle groove goes on the side the thread goes in. There is an arrow on my Singer 404 but it goes front to back. I hadn't noticed the arrows before last week.

That's useful to know. Thank you Miriam.

Clare

LOL. I'll have to check for arrows.

They don't ALL have arrows... I wish.