Rocketeer

Hi Everyone,
I've been searching for a bit and can't seem to find the answers to some (I think) pretty basic questions about vintage machine needles, so I thought I would ask those who would likely know and/or be able to direct me to the appropriate resources....
I'm trying to figure out what needles are what, and approximate ages, based on the inscriptions on the shank. I've obtained a bunch of vintage Singer needles to fit the machines I'm interested in using (15, 27, 500a) and my goal is to know what needle is best for what material (or machine!), or when to choose a particular needle.
So far, I've separated the needles I have by the following inscriptions. I have been able to find some correlations/explanations, but nothing comprehensive:
75/11
80/11
90/14
11GY
14GY
16GY
18GY
and plain 11, 14, 16, 18

In particular, I can't figure out what the "GY" suffix refers to. Any explanations would be so helpful! I suspect some of these became the "color banded" identifications of the more modern packages. Also I think the 75s are ball points.
Finally, I think I have the old packets arranged in order of date... does anyone know approximate dating for the individual package styles?
Thanks so much!
Matt