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If there is no visible barrel it cant be M7 nor M8 which got MG151/20 guns.
M6 is known and got a different kind of balken cross on the fuselage.
M5 was used for destructive test and never flew.
M3 and M4 got the downward wingtips. (but perhaps not for their first rollout?)
So imho, M2 W.Nr.200002 looks to be the best bet.
Another possibility is that among the 20 pre-production order W.Nr. 220001 to 220020, the first one was complited before M6 and so could also got early wingtips and early balken cross, obviously doubtful.

Cheers, olivier.