Odd. Last combat mission of both 1st and 3rd Gruppen seems to have indeed been on april the 28th, apparently under the command of an ensign(not quite appropriately translated from the German Fahnrich, I'll admit) Wichmann. By that time however both the gruppen were no longer even close to Prague, but apparently operating from a place called Bad Aibling, a place where they had been since april 20th.....
http://www.amazon.de/Chronik-Jagdgeschwader-27-Hans-Ring/dp/3613034611/ref=sr_1_5?ie=UTF8&qid=1403274650&sr=8-5&keywords=jg+27
Thats what I found there...