Yeah, if you're using an aftermarket carpetIt's hard to identify anything from the inside of the shell. Lugs would make things easier. This is like trying to identify a car by it's carpet. We need more info.
I did think the shell I have is either Ludwig or DW. Since DW has badge that's held in place by 2 small screws, I discarded that option.This is the interior of my 80s Ludwig marching snare. I'm not saying you have a Ludwig, but the plies look strikingly similar.
The grain pattern on your shell looks fancier on the interior. I'm not an expert on date stamps, but I don't recall ever seeing a stanp with that font before. Especially not a major manufacturer.
Hm, PDP seems to have smaller distance between lug mounting holes.I think it's safe to remove DW from you list because it doesn't have reinforcement rings. It could be a PDP Shell, but I don't think they made 18" tom's until fairly recently.
What finish does/did it have?
Son of a gun, I also checked with good Premier people, but as I removed the lug screws, I noticed the original lugs had two holes so it's not a Signia.The key distance would be between the outermost lug screw and the empty whole since that probably accommodated the original lug.
Good eye! It's camera trickery. The finish ends right on the apex of the bearing edge. The ply that's after the apex (when looking towards the center of the drum) is not as dark as the stain finish.Maybe it's just me, but I'm skeptical about that being a factory finish. It's on the bearing edge and doesn't appear to be even from the pic. Not even meaning in some spots it looks like the finish covers more of the bearing edge than others. This could be camera trickery though.
After reading your post I googled Sonor, but they have totally different stamp that marks the year and month of production all combined in a 3-digit stamp.I can't help you identify the shell, but the date stamp format leads me to believe the drum's origin is Europe, perhaps Germany. Decimal dating is a common format in Germany.