In FAA land there is only one ATR type rating that covers both the 42 and 72. Guidance is found in AC 61-89e. While old, it is still current. The "600" is covered in the Type Certificate Data Sheet A53EU. What it says is, there is no -600 model.
(a) ATR42-600 Definition (Approved April 4, 2013)
The ATR42-600 designation does not correspond to a model designation. This is only a commercial designation for an ATR42-
500 on which Major modifications 5948, 6521, 6230, and 6233 have been embodied during production.