The use of just one or two relatively large cymbals as both a crash and a ride was pretty common at that time. Look at most of the jazz guys from back then and you'll usually see a pair of 20" cymbals or a 20 and a 22 in use both for riding time and crashing.

Mel Taylor of the Ventures had just one cymbal (looks to be a 20") that he used for both so maybe Dennis was just followed the trend.

A 20" A. Zildjian Medium Ride in the 1800-2200 gram range will usually work quite nicely as a single "crash/ride" for this sort of a setup.

I'm profoundly left handed. I set up my first drums lefty style. I was uncomfortable with that configuration. After a few weeks, I switched over to playing with lefty feet and righty hands. That lasted for the one gig in the picture. Then, I tried a typical righty configuration. I gravitated toward not playing cross-handed on hihats. I also set up mirror image ride and crash cymbals on both sides of the drums. If my right hand got tired, I'd switch to lefty ride. I had no idea what open-handed playing was until I did it for many years. I stil play with righty feet and either lefty or right hands. I call myself the slowest bass drum foot in the West. I use a very similar set up to Dennis for the drums, but more cymbals than he used.