I don’t know for sure, but I think it might take a 20x1 type needle.
The machine on your manual appears to be one of the machines that was designed by the Portman Sewing Machine Company, and called by several names. Seamstress, N-50, and Model J are a few of them. I have a Montgomery Ward version of it that’s either an N-50 or a Model J, I haven’t ever figured that out.
CD in Oklahoma