I used to machine these, albeit the later flat cam cover ones.
I am pretty sure it's oversized cam bores. Cam bores were line bored using adjustable inserts, tappet bores were machined using fixed blade Mapal guided reamers, any over size tappet bores meant the casting was scrapped.
The letters are probably the initials of the person who did the machining then the line boring, doing this was a requirement after every big fuck up but would only last a few months.
Out of interest, up until about 1990 the machining was done on Moog tape controlled vertical milling machines, after that on Mazak 3 axis vertical machines. If you have flat cam cover carriers with the I and E letters stamped on they were done on the Moog, if it is machined on it was done in the Mazak. I wrote the CNC code for the Mazak machine to machine these.