Hi Steph,
firstly let me congratulate to you - you are the first modeller I know (who confirmed) who camouflaged P-39 kit according to my profiles
Now seriously. You are right - VVS standard during WWII was without red stars on the upper surfaces, therefore Russian manufactured aircrafts (La-5, Lagg-3, Yak-1...) didn't wear red stars on upper wings at that time.
And yes, there are photos of Cobras without red stars on the upperwings - like one on the photo above from April/May 1943 on the North, airfield Vaenga - P-39M-1 "02" s/n: 424939 from 2 giap, latter transferred to 255 iap and renumbered to "14".
On the other side on this photo from May 1943 there is red star without white outline on the (very probably new) Glinka's Cobra:
Same as on Pokryshkin's "100" s/n 42-9004.
Anyhow, as there is not hard evidence about Glinka's "21" it is a matter of personal prefferences/decission.
Decals of the number "21" and victory stars are home-made?
Regards,
66misos