The guts may be new, but did you replace the handles? Also, as mentioned, all new valves add some protection with anti-scald technology. This may or may not be an issue, but it will certainly make it easier to sell the house when the time comes, and if there are any elderly or really young users of that shower/tub, then they are more susceptable to getting hurt if there's a momentary surge of hot or cold water...the new valves protect against those major temperature changes.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014
I fixed it!!!! I went to a local Plumber supply warehouse and they told me they did not carry the part but they gave me a card of another plumber supply store which upon going to, within seconds of walking through the door and showing the gentleman the part he identified it right away and said they had the whole assembly seat handles and all the stuff so he asked if i needed the whole thing or just the diverter... he gave me the parts which matched exactly, it did not have a label on it and I still have no idea what brand it was but I came home right away installed it and it works! no leaks! way to go Kirk Plumbing in El Cajon California! they have a huge variety of parts old and new, If you're in the area or stuck looking for an exact part im sure they can probably help... website = http://www.kirk*******************/ Thanks Everyone for the advice and help, it is very much appreciated!
Neither the Central or Gerber BONNETS look anywhere near the same as yours, and that is the thing that DEFINITELY has to match if it is going to fit the valve properly. The center stem is almost irrelevent as far as matching yours, because once you get the bonnet right, the stem just has to screw through it properly. Which it will if you get the whole assembly. Your handles look EXACTLY like Harcraft, although there are probably other similar ones.
Me thinks it might have been a sterling...