Hoop, it was the same part number 77 6050A. Apparently, there would have been two supply lines of heads at the engine assembly, one drilled and one not drilled. This practice continued into the 24 stud production with the 81A 6050A. The 81A 6049A was released in April, without the boss at that location. At some point, the boss again appeared on the RH head, and began being drilled for heater fittings in October 1938.
As there were a number of additional heads for different purposes throughout the period, you can see that the added confusion of the hole drilled or not drilled was a headache that didn't go away. The hole itself was different if for heater fitting or temperature bulb, the latter having an inner boss for the bulb. The bulb was finally redesigned to not require the ledge, but then needed an adapter if fitted to the early hole.
It is these kinds of minor differences that are difficult to understand today until seen in comparison to unrelated developments, where one small change could affect several others.