'66 master cylinder differences?

I bought a replacement master from napa and its a little different. The replacement has a brass plug blocking a second opening on the underside of the master. As you can see in the picture the original only had the one opening and a solid bottom.

I know in '67 they switched to the safer two piston, dual reservoir design with two lines coming from the master.
Did some cars have two lines coming from the single reservoir masters in '66?

Hey Flyboy,I cant tell you what to do but this is what happend to me.I have power drum brakes on my 65 Belvedere ,and when i first bought it i replaced the m/c .It also had a brass plug ,and it didnt really bother me.I just installed it and away i went.Maybe you could ask your local mechanic ,or a brake shop and you may feel better about it.Im in the middle of switching my car over to the dual m/c.
Paul

thanks for the info, im not really too concerned about it. Just wondering if it was something factory or if it was maybe part of the reman process. I plan on eventually swaping over to front disk and dual resevoir/piston master before the 440 goes in, but thats still probably a year away or so.

just an update, installed it and sure enough that bottom plug leaks. Ive tightened it down as much as it can take, any more and im pretty sure something will break, and it still leaks when pressure is built up in the system. I guess i'll take it back to napa and see if they can exchange it....now i see why people tend to talk down about reman products....

There should only be one line coming out but maybe there is an application where the line needs to come out the bottom (perhaps a big truck). This master with two ports probably eliminates the need for two separate part numbers. Before you go stampeding back to the store, remove the plug, clean the parts and put a little pipe dope around the threads. Not too much and apply it about two threads back from the end.

That master cylinder fits more than one model year, the older models had the brake light sensor pressure switch there instead of under the dash off the brake pedal

I have the same m/c that you installed and had no problem.Must be easily fixed

Why not just upgrade to a dual master now instead of messing with that?

That is good advice ,im in the process of do so myself.Dual m/c