Help with early Schönberg works

Hi! Do we have anyone who knows a fair amount about Schönberg here? I’m trying to figure out this album:

At first I thought the bits like U10 were some sort of catalog numbers, but now I noticed that several seemingly unrelated pieces seem to have U10. Does anyone have any idea what this is actually supposed to mean? And, more generally, how the hell should the works for this album look like? :slight_smile:

Maybe it relates to manuscript paper type and/or sketch bundle:


That site looks like a good source to figure out what the works might be, at least :slight_smile: Thanks for that!