Plug for 71-72 combination/proportioning valve

[dmjlambert]

This looks like a bleeder or zerk to me in the right front brake port of the combination valve. Is that what it is? Is this sort of plug what was used from the factory, or does this happen to be what somebody used on this particular combination valve? Is it normal? it looks like a plastic plug in it.

I understand this is the port to right front brake for some vehicles, but this is plugged in the 71-72 C10 truck application