Your frame is interesting, and i'd love to know how it was originally equipped. The fact that it had a triple crank makes me think it was sold as a mountain bike, and it might have been old enough (or cheap enough) to have used 1080-style calipers rather than cantilever brakes... but getting a look at the original bars, stem, etc would tell us a lot more.
To me, the original klunkers were old cruisers that were modified for offroad use; John Finley Scott build his first re-purposed Schwinn for trail duty in the early 1950s, long before the rest of the Marin guys had even begun to experiment with balloon-tire cruisers. JFScott put knobby balloon tires on lightweight bikes and ran 3 speeds with gearing far lower than the manufacturers suggested. He called his machines "Woodsy Bikes", but to me, that's a diamond-frame klunker b/c he was taking a bike designed for a different use and modifying it for trail-riding. I do the same thing with various industrial bikes, and i consider it to be "klunking" b/c they're bikes with low- to no-tech being repurposed for offroad use, but i know a lot of "purists" would say it's gotta be a pre-war bike to be a real klunker.
Whatever; the MArin guys were certainly not purists-- they rode strictly Frankenbikes, running whatever parts they could find that would make the bike more viable on the trails. They preferred pre-war Schwinn DX and Model C frames, b/c they were built to last, and had nice geometry, but they also road postwar bikes and they were happy to move on to purpose-built bikes as soon as they had the money, tools, and know-how.
I've klunked a Schwinn lightweight or 2, and i'm about to build up an old Schwinn racer as a bomber-style klunker... I guess, if you can klunk it, it's a klunker...
