You should probably post pictures. However, if I had to guess that was a designation somebody was using to organize their collection.
I will try to post some pics when I can. Some of the white square have lower case y and then a # after. It could be my Uncle was numbering them himself. But with thousands of coins in individual packages, I have zero idea. My head is still spinning.
I agree with @tommyc03 , sounds like your uncle was using Yeoman numbers (a cataloguing system for world coins, which has since been replaced by KM [Krause-Mishler] numbers).
Ok. So, what is Yeoman numbers compared to KM? My Uncle was in no way a savvy collector. So, I still sit here not knowing what any of that is. As stated, I am a newbie. Virgin if you will. I have never been into the coin collecting world until I got most of his stuff.
I'm not really sure what happened to R.S. Yeoman's catalogs. It's possible they did not keep up with the times and plethora of coins being issued. The difference would be that the Krause/Mishler numbering system became the standard for world coins including U.S. so unless you have access to an old Yeoman catalog the numbers would be meaningless. I don't believe there is a cross referencing published but maybe someone here knows more about it and how you could do this.
You are correct but they did include several, maybe 4 grades in their references along with values. But not to be confused with something like a grading guide.
The lower case "y" stands for "Yeoman", the publisher of a guide book of world coins. The # after the "y" is the catalog number assigned to that coin in the guide. It will be intimidating trying to figure out all those old numbers and how they cross over into the Krause guides. If you are talking about mainly world coins you will need to start from scratch unless you can locate an old edition of the Yeoman catalog. If it's U.S coins you will only need a Red Book Guide To U.S. Coins.
I believe most coin books do not list cross-references to other books; at least the Krause world coin books I own don’t. However you can search online and may get lucky. E.g. Cuba 1 centavo: https://en.numista.com/catalogue/pieces1017.html References KM# 33.2, KM# 33.3, JMA# 20-26, Y# 27, Schön# A31
