While looking for other parts at NAPA I was able to ogle the parts book (why are these things so damned hard to get?) and found the following items that appear to be very hard to find. I don't know if you can actually get them but what the hell, ask:
NON-RAMBLER APPLICATIONS: 1974-1992 Volvo 660-1286 thermostatic (correct part) 660-1067 non-thermostatic (not correct; works, but OFF/ON reversed)
The latter valve (72-73 Ambo) beats not having a valve. Cable push vs. pull on app. car makes it backwards on my 63 Classic.
Thermostatic (and similar) RANCO repair kit: 660-1000
This kit is sold by our AMC businesses, but NAPA apparently can get it. Fits later-than-Rambler Volvos.