I am not sure when exactly the transition was made or why, but I've narrowed it down. I checked out a nice original 40 motor and it had the early style. I saw a restored 42 on Matt Olsen's blog with the late style (beveled). I have two 41's and one has the late and one has the early. I am sure that Matt Olsen could clear this up for me. I originally figured they changed when the pushrod tubes changed in 1940 but that does not appear to be the case.