Wrong master cylinder in my Dart?

Started working on a '68 Dart 270 that I hung up and neglected about a decade ago. It's got a 273, 904, and manual 4 drum brakes. I noticed before I stopped driving it that I'd periodically have to adjust the rear brakes because they'd start dragging and heat up. Never investigated any further.

Fast forward 10 years. Now I need to replace the master cylinder (leaking all over, no pressure). Problem is the piece that's installed looks different from those online or at the parts stores. From what I've seen the replacements have a single bolt going into a flat metal cover over two square chambers.

The one I have has two identical humps and a wire holddown over two rounded chambers. Looks more like those on manual disc Darts or '71+ Darts with manual drums. It looks better but I'm wondering if the previous owner installed the wrong one. Are they supposed to be interchangeable? Could that have been my problem with the rear drums?

I found some threads on folks upgrading their brakes to disc or power but I'm trying to keep it stock. Only reason I'm asking is because if I replace the mc with the "correct" one then I won't have the proper core for exchange. Not a big deal ($15) but thought someone might know if it makes a difference. Thanks for reading all of this.

I have found there to be a discrepency between right and wrong at the big chain parts stores. What enthusiast isnt tired of answering pointless, mundane questions that do not matter?...Yeah, I need a master cylinder for a 68 Dart with manual disc....." Uh, what motor? Manual trans? Power steering? 4 Door?... NONE OF THIS STUFF MATTERS ! The majority of parts counter guys are drones that know less about cars than my mother. They were trained to use a computer and to help old ladies install wiper blades on their cars in the store parking lot. MUCHO respect to the pitiful few competent parts guys still in the business. They are smart enough to bypass the unimportant questions when doing a parts search. Okay, on to your situation. Are you sure that either of those new MCs are correct? On a disc car, power or not, isn't the front brake reservior supposed to be bigger? It seems that all the dual circuit disc MCs I remember had the big reservior nearest the firewall.

Thanks for the response Frankenduster. From what I found on the Checkers/O'Reilly website it seems that the only the power disc applications have MC's with the larger reservoir in back for '68 Darts. I also checked Rock Auto and Napa online and the pics look the same.

All that seemed to change from '71 on up. That's why I was wondering if perhaps someone installed a later model MC on my Dart by mistake (or on purpose!). I guess I should just bite the bullet and go with what the catalog says.

The parts guy happens to have a '73 so luckily he's not totally clueless but he wasn't sure about the interchange with my year.