Master cylinder leaking?

I had one a single pot w/ both a front and bottom port, as you state. Might have been on a new booster I bought for my 65 Newport, and think I sold it. I now run dual reservoir MC's on all 3 old Mopars. Teflon tape would work fine. Main thing is to never remove the plug since little shreds of teflon could get everywhere in the brake system. Also, don't over-tighten the plug. Those are tapered pipe threads, so overtightening will easily crack the cast-iron body. I had that on the single port of one such MC and had to fix it w/ epoxy. Probably the crack was already there from the rebuilders since I am careful.

Re what would have used the dual ports, perhaps the PN crosses to other manufacturers. I think GM used 2-bolt MC's in the 60's, so not them. What about Ford? Some MC's today have a separate port for each front wheel (rather than a distribution block). My 80's M-B is like that, as are 90's Chrysler "cloud cars" w/o ABS (why ABS MC is better for my classics). My M-B cars don't appear to have a F-R pressure imbalance switch like 70's Mopars do, so perhaps that wasn't a fed mandate or was dropped.