Hi Tom,
Nick Beale's excellent Ghost Bombers site states that Sommer was St.Kap. 3/JG2 for his 29Feb44 claim and at the time of his death on 14Mar44. Nick's work is very well researched, data is usually from primary documentation.
http://www.ghostbombers.com/JG2/jg2frame.html
As for the claims for 29Feb44 and 03Mar44 being listed as Stab.I/JG2 maybe Sommer was also acting Gr.Kdr. at the time (Erich Hohagen Gr.Kdr. I/JG2 at the time is conspicuous by his absence of claims for this period).
Maybe Nick could shed some light on the matter.
Best Regards
Andy Fletcher