RLM66 was indeed mandated by an order in 1941 (November I think) and this is a good guide line. There is plenty of evidence for its use in interiors much earlier. Certainly early Ju88s in 1940 had cockpit interiors in 66 and not 02 and they were not alone.As with all things Luftwaffe try and find a good reference for your aircraft or ones built around the same time by the same contractor. Different contractors were interpreting orders diffrently!
If all else fails fall back on the guide line.
Look on the bright side, noone is right or wrong.
Cheers
Steve