Do Proportioning Valves Wear Out?

Okay.....this has become one of the more frustrating sourcing situations I've had since owning the 69.

Can someone tell me the difference in the two proportioning valves below please. InLine is telling me the valve doesn't fit the 75 Dart brakes, while Classic Industries says theirs does. I can't see a difference.

http://www.ebay.com/itm/39048620718...&_from=R40&_nkw=390486207187&_rdc=1#vi-ilComp

http://www.classicindustries.com/mopar/parts/md2268.html

Thanks