Thank you Andrew! Yes, you are absolute right. What I meant to say was that maybe the "black L" in which he scored his fifth kill and the aircraft in my first post are the same machine considering the curious over-painted L, the second group bar and the "Indian head" marking (not a F-8 but probably a A-5 or F-3 and maybe stayed with the II/SG1 after 6. Staffel's move to SG2). But it is, of course, a speculation. Anyway, building a model of "black X" would be a better option since he both completed his 600th mission and shot down 6 planes in it as well.
Best,
CZ