Okay, I have a plausible explanation now
In charge of Reichsgau Danzig-West Prussia was Gauleiter Albert Forster who went about Germanising the territory by indiscriminately registering thousands of Poles as Volksdeutsche regardless of any German heritage. So much so that his counterpart in the other annexed Polish territories complained to Himmler that Forster was registering everyone without regard to racial policy. Himmler naturally ignored the complaints.
Forster's Volksdeutsche registration policy could explain your grandfather's conscription into the German army.
I think the abbreviation "T.O.P" could be "Tajna Organizacja Pomorska (Secret Organisation Pomorska) ” a very shortened abbreviation of Tajna Organizacja Wojskowa Gryf Pomorski. (Pomeranian Griffin)
The Pomeranian Griffin organisation was created on July 7, 1941 in Czarna Dąbrowa near Bytów.
The Pomeranian Griffin's leader was a Lieutenant Józef Dambek, and he was killed by the Germans in 1944 so this could be the reason for the 1944 date and not the Warsaw Rising as I first thought.
There was a TOW or Tajna Organizacja Wojskowa (Tajna Organizacja Wojskowa)
All supposition I know, but I think T.O.P - Tajna Organizacja Pomorska is a plausible explanation to the armband.
What do you think?