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.