All of them from the beginning in 1958 came with the cocking indicator. I am not sure where the cutoff line is with that option.. You can also go to Rimfire Central which has a date list for the M41's on their site as well. Myself I do enjoy having it on my 1958 year m41 a lot.
I believe that the M41 stopped coming with the Cocking Indicator around 1977 - 1978. One of my friends has one made not too much earlier than mine and his has one, I don't. Mine was made in 1979 and his about 1977 or 1978.
Not precise, but that should put you in the Ball Park - sorry can't look it up right now - going out to dinner.