Right side was oil overflow breather. Only P-51D I saw that had a similar port on the left side wasn't an actual P-51D. It was a P-51B modified to test the canopy that would be used on the P-51D. The NA-106 P-51Ds may or may not have had it, since few (if any) were made aside from test planes. They were superseded in production by the NA-109 series produced first iteration P-51Ds (provision from the start to use the 85 gallon fuselage fuel tank), and I don't remember them having the left side coolant port.
It should be noted on the P-51H, XP-82 and P-82B that the oil breather was moved further up towards the spinner on the lower engine cowling, above and between the engine filter air intake the the main supercharger intake. There was no similar hole on the left side, as well.