Raven is right: numbers molded into the plastic are MOLD numbers, and are the same for parts of the same mold. Completely unrelated to part numbers, which differ with colour and printing. I do have some in the database - they're searchable via text search - but I only have a few hundred, since I have to have the part in hand to be sure of the mold number (and only large-ish stiff plastic parts have them, it seems).
About the tray: looks like a mistake to me. I don't have a definite answer about its origin, but I'll explain my detective work.
If Raven has this part...
Part 30 21 8003 - "Picture frame, flat back" in blue from
5496 - Advent Calendar Christmas Room with Illuminating Treeand it is from the same mold as the mystery brown one, then it may be that it has the same German name: overwhelming evidence to support this. Other parts with
"Leinwand" as the German name are:
Part 30 45 4480 - "Picture frame, flat back" in white from
5998 - Playmobil InnPart 30 21 5023 - "Tray with raised edges" in light yellow in
5587 - Christmas Market(There are a couple that are clearly not the same thing, but we'll ignore them for now.)
There is also a part that I have a number and German name, "Leinwand", for, but no picture or set assignment.
Part 30 21 6043 - "Leinwand III" in ?This may very well be it. The sequence of part numbers starting with "30 21" leads me to believe it was first released in 2014. If anyone, digging through 2014 offerings, finds it, I'd be happy to match it up. I'm still pretty sure it doesn't belong with Dürer, though.