Most of the photos of a/c with large white numerals on the tail (and sometimes on the upper wing also) are mostly a/c from the IVth Gruppe(Erganzungsgruppe) but these numbers can indeed also be seen on a/c from the other 3 Gruppe.
In my opinion this can mean 2 things:
- a Gruppe that temporarily was redrawn from frontline service applied these numerals during training. Most specifically during formation flights.
- Experienced pilots that were temporarily reallocated to the IVth Gruppe to pass on their knowledge, brought their “own” a/c with them.
I have seen numerals in the #50 & #60 range so they have nothing to do with the Staffel. I was always (untill now) under the impression that these were the 2 last digits of the WNr.
Hope this helps.