You could be right, it does not really have the exact shape of this 50th year PLM crown, but its a far cry from an oak leaf or anything else....and the rest of the PLM is really a rough sketch, so a crown may indeed have served as the model for it after the award was handed out some fifty years later.
My point on the artist was not that it was done during the war, but instead that the artist that did the other sketch too, dated it 29-8-21, which I assume means dd/mm/'yy 29-08-21 or 29th day or august in 1921 was somehow around to do the second one. Even if the PLM was done after his reception of the 50th year crown, he seems to have had a long relationship with this artist. I just wonder who it was. It may be a stretch but I got nothing but straws here anyway.
I'm interested in any angles you can provide that may help.