Help With Proportioning/Combination Valve Identification

[cericd]

Got this prop valve from another member with the intention of having it rebuilt and using it on my 1970 C10 to which I've added factory style 1971 front discs and a booster to but I'm wondering if it will work for my application. Seems the factory ones I've seen have a rubber cap on the front. This one doesn't, just an indention and smooth metal. I'm told it came off a '71. Trying to figure out if I can have this rebuilt and use it or if I need to find another one. Here are some pics of the valve and the only pic I have of the truck it came from. Any ideas?