None of the 3.0 930 turbo heads had RR350 marked on them, they had Y.
3.3 turbos got the heads made from RR350 during MY 1978.
My 1978 turbo has been made autumn 1977 and letter Y is casted on the heads. 930 heads made January 1978 have RR350 on them. I don't know the exact month when did the change happen.
edit: or maybe Porsche decided that they have to mark RR350 on the head and this was the reason for changing from 930.104.341.2R to 930.104.341.3R but they both could be from excatly same RR350 material.
edit2: I started new thread in here
930 3.0 and 3.3 heads
934 & 935 had letter Y on their heads, at least the earlier ones.
Actually the 935 heads on the link below has the exact same cast number as on my 930 heads 930.104.341.2R and that 935 head has been casted July 1979.
935-twin-plug-heads-imsa-champions-john-fitzpatrick