I don't think I can help much about the needle denomination... I only think they changed the classification names and numbers a couple of times and that there is a difference in size names between Europe and America...
If you want to make sure the needles fit in your Singers (15, 25 and I think 500 all take regular 15x1 needles) you can take a modern needle and compare the length...
As for dating the packets, you can check the logo, on this page down here, the lady shows the Singer logo evolution
http://www.lamachineacoudre.net/arch...tion_logo.html