Cut Squares

lots of cut squares with a you no. and letter like a b or c whats the magic system for those letters

I think you're referring to the Scott Catalog numbering system for postal stationery items.

Everything that's postal stationery (envelopes and cut squares) begin with the prefix "U" (or "W" for wrappers). Likewise, postal cards (or stamped cards, as the USPS now refers to them) begin with the prefix "UX" (or "UY" for reply cards).

The suffix letters (lower case "a", "b", etc.) refer to essentially the same design envelope, but are sub-varieties featuring small detail variances in either the design, die or paper type/color used.