I know the originator of this discussion asked that the topic be closed but I think the Numista webpage for this coin series may still have a problem related to this issue.
Before discussing that, the coin that instigated this discussion isn't real. The mintmarks on the coins in this series were raised, so a skilled metal worker could probably remove a mintmark from a normal 1928 dated coin in such a way that the alteration could be difficult to detect. The coin in question, however, has lots of design deviations which brand it as not real (either a contemporary counterfeit, or a modern fake) including the style of the large "50" on the reverse, and the details within the oak leaves and acorns surrounding the inner circle on the reverse.
I think we can absolutely say that the pictured coin does NOT prove that some 1928 coins were made without a mintmark.
Now here's the problem that I wanted to bring up related to this issue: On the Numista webpage for this coin series, there is a comment that (1) repeats the list of German mints for some reason; and (2) includes the following cryptic statement: “The year 1928 also exists without a different number.”
The original comment was in French (and this might just be a bad translation - the apparently “mints ” in French are “monetary workshops” - but does anybody know what this statement is supposed to mean? When I first read it (before I read this forum discussion), I thought it meant that some 1928 dated coins in this series do indeed lack a mintmark - and that leads back to this forum discussion. I don't know if the comment was added because of this discussion, but since it repeats information already present on the web page (the names of the German mints) and because it might be implying that 1928 dated coins with no mintmark actually do exist, I recommend that the entire comment be removed from the web page.