Gruß Gott Kari!
Well, it seems now that we have an ongoing mystery plus a difference of opinions!
I cannot explain the J U + X Y code references to a Finnish AF Bf 109G-2 that you have found, it seems a mystery to me. In no way wish to argue, but I must repeat that based on the patterns which appear in the tens of thousands of SKZ references (loss reports, NVLM, Flugbücher, recovery reports, factory documents) as well as thousands of photographs that I've seen, the Luftwaffe did not issue SKZ beginning with "J." Of course, it's statistically possible that only a tiny number of SKZ beginning with J were assigned, but it seems highly unlikely to me that at least one of these has escaped notice, especially in a photograph.
The SKZ did not necessarily have to be painted on the outside of the airplane, especially if it had been repaired. SKZ were often painted inside the airframe, particularly in the cockpit area. SKZ also frequently appeared on compass cards,
Lebenslaufakten, factory documentation, and other documentation. . . . But you already know all of this!
I looked at my files, and the LwGQM daily loss report for
WNr. 10 357 on 6 Nov. 1942 does not record the airplane's SKZ. It is also interesting, isn't it, that Keskinen and Stenman did not record a SKZ for MT-217 in their monograph on the Bf 109G in Finnish AF service? Even the new edition of their book doesn't have a SKZ entered for MT-217. . . .
Kari, the approximate date(s) of the movement of this airplane to Finland should be known. Is it possible, therefore, that you have access to the Malmi A/D, Helsinki, Finland, register of transient aircraft? Does a Bf 109 coded J U + X Y or B Q + O M or B G + O M appear in this register?
It seems to me that we must call on some other opinions and/or outside help here. Peter Evans, Dirk Lehmann, or Tom Willis, are any of you listening?!
RA