The reality is: Vintage detector collecting is just not wide-spread. Little to no demand. And the late 1960s to early 1970s was a "zenith" of the hobby. Lots of various machines sold in those years. So they're common to find at flea markets, garage sales, ebay, CL, etc...
Sad: Because yes: some might in fact be hard to find. But for the big manufactures (Whites, Garret, Fisher, etc....) there's no shortage of machines from that era on the curiosity-market. And I venture to say that d-tex is also one that's not hard to find. And to the extent that it might not pop up too frequently: As I say, there's just not that many seeking them.
IMHO, the really sought after valuable ones (if there were anyone hard-core collecting/competing) would be machines from the late 1950s that were capable of finding individual coins (verus 2-box or military mine detectors that could only find hub-cab sized stuff). There were a few rogue machines in the 1950s, that could find objects as small as coins and bullets. THOSE machines would be cool to have.