Do not own an "A" series machine, but i can tell you about the drain plug on the vertical head on the FP-NC machines.(the "A" vertical heads are different from the FP-NC vertical heads)
So the story goes like this:
The operators manual for my FP4NC (1st generation camel back vertical head) gives a detailed description of using the drain plug on the vertical head(located above the hand quill feed dial , as part of the casting boss that
holds the vertical tram plunger) ....It gives a procedure where you tilt the vertical head, remove the plug and fill the cavity with oil to the bottom of the plug drilling....Provides lube to the drive gears in the vertical head...
When i bought my FP4NC in 1993..it came from Germany (bought used directly form the factory) It came with a warranty and arrived with a factory technician, Volker Spitz, to do the install.
The question on the oil to the vertical head was addressed by Herr Spitz....He told me to forget the manual instructions, and not to fill the cavity with oil...Rather i should remove the plug every 30 days, and tilt the
vertical head to point the plug drilling straight down, and allow any oil in the cavity to drain.....
I was confused by this instruction and asked again if i understood his instruction....He was quite clear. "Drain the oil every 30 days of operation"
There is oil fed to the gear (metered small amount i am led to believe, witnessed by the fact that oil keeps appearing when i remove the plug.)
I was told that there was a factory service bulletin that dealt with the change in the lube procedure for the vertical head....
That a small amount of oil was all that required to lube the bevel gears.
I have been using this procedure since i got the machine (1993) and as far as i can tell all is well with the gears and bearings...Further, think failure to drain the oil will eventually result in the oil getting high enough
in the head to enter the spindle bearings and wash out the grease....
Additionally it should be noted that the late Flip head FP-NC's were designed with a drain built into the vertical head to allow the oil supplied to the bevel gears to return to the sump of the "Y" slide.
Take the above information with a grain of salt, as said before, your vertical head is somewhat different, and may have different requirements...
Cheers Ross