Ted;
They must not have been Singers. I do not believe there is a single screw on a singer than will match any standard of today's screws. Many have stated this was to force people to buy replacement screws directly from Singer. Fact is these screws were all made to suit a purpose long before there was a Standard. By the time there was a standard they had been in production so long Singer felt it in their best interest to continue with the same sizes rather than having different sizes on older & newer machines. The threads per inch are all regular but the diameter & TPI vary from today's standard. There were no Numbered sizes at that point in time such as a #6-40 etc. Singer did use a 9/64-40 a lot of places which is .002" larger in diameter than a 6-40 so those will sometimes interchange depending on tolerance
