[Karl]

Dear David, Nick et al,

I agree with you that "Red 1" and "Red 2" do not fit in the marking scheme of KG51.
Jan Horn in his book "Das Flurschaden-Geschwader" (available only in German) offers us an answer. This may be planes of the "Versuchsschwarm General der Schlachtflieger (G.d.S.)", an experimental unit integrated in I.KG51.
Known members of this group were Major Brücker, Lt Strate and Fj-Fw Fröhlich. These people left I.KG51 at Memmingen on 23th April by car and joined JV44 at Munich-Riem. They can be found on the JV44-pilots list of 27th April 45 with the remark "G.d.S".
Interestingly, Fj-Fw Fröhlich left JV44 and joined 2.KG51 again to fly from Munich-Riem to Prague-Ruszyne on 29th April. On 8th May he flew from Zatec/CZ to Fassberg with well known "Black X", now on display in Australia.
One proof that the G.d.S.–planes were marked with red numbers is the following. An ULTRA decrypt reports, that on 11th April 45 Lt Strate made an emergency landing with WNr. 170 053, "Red 2" at Hollenbach near Neuburg/Donau.

@Franck: David was a bit faster in showing the photo you wanted to see.

Karl