[rotorwrench]

The lower compression heads were used on Mercury cars to get the best burn with the 4-inch crank for the fuels of the day. There were 8CM heads for 49 thru early 51. The 1CM head is a replacement for these that you will find now and then and may have graced production engines thru the end of 1951 production. The EAC heads were late 1951 through the end of flathead Merc production. They all have "Mercury" raised casting letters that identify them further. For "Original" Mercury cars, the appropriate Mercury heads would be desirable. For a hot rod motor, not so much.