Why some games have umds without umd letters on it?

[RadekNeli1987]

Ok I made some research and it's as you said.
I had a theory that later releases of same game version do not have this letters (like they started to produce with them but decided to make without them later to distinguish original from platinum and then all essentials also do not have them) but little big planet (game from 2009) have platinum version with and without them, so it doesn't matter because it's means nothing.