Correct you are
Fk is Fokker
Dr.1 Is the model The Fokker Triplane, design "borrowed and enhanced" from Sopwith of England
321 is the most recent incantation of the plane, Mine!
There have been about a dozen or so Dr.1 replicas since 1918. None are known to have survived WWII. An original pair of them was destroyed in Germany when a museum was bombed. I saw one flying at the Old Rhinebeck Airdrome in NY State as a kid. That was 40+ years ago and it still gives me goose bumps thinking about it.
I was going to use Lothar's plane designation, but that would have been really to obscure. Besides, I couldn't put Snoopy doghouses on the side as kills then!