It all started with a phone call from an enthusiastic forum friend informing that he had just bought something that he couldn't clearly identify. In the poor quality first photos, probably taken in the basement of the previous owner, you could see a patchwork of parts from the 601 OSL (closed frame) and 501 OS (typical front fork, aand drive unit with a cast iron head). More photos started to arrive, taken after the motorcycle was delivered to its new Polish owner. Soon it was possible to catch a few characteristic details that prove that the body (frame, fork) comes from ... an extremely rare and valued SS sports model. And it was already a sensation, but then more commercial than substantive. So far, the few SS-s have rather left us to the west than vice versa, which led to their complete extinction in our area. So few people here today know about this rare and expensive model.

Only the engine with a tappet valve timing instead of the SS king shaft drive did not met. Hot minds have already seen a sport competitor who, perhaps in the difficult post-war times, built a vehicle for racing or rallies himself from available parts, perhaps in a harness. But then one important detail appeared - the crankcase had a stroke of 105 mm, so 6 mm more than in the typical 501 OS/OSL engine or the related 601 OSL/OS-WH. This larger stroke was later confirmed by measurements of removed engine components. We realized then (among people interested in identifying this motorcycle) that we were dealing with something extremely rare, unusual and not shown in any official factory materials, at least as far as the engine itself is concerned. Unfortunately, the original frame numbers were obliterated a long time ago, which made it impossible to identify the body in the factory registers (via the Audi Tradition office), so it must be recognized that the originality of the entire set is still not definitively confirmed. But the engine itself is sensational enough.

The search for information, discussions, memories of the stories of the "old riders" began. A friend who still remembers the realities of the 70-80s associated that he had heard about such a model and even saw similar frames or engines, but never in the form of a complete motorcycle. As the discussion developed, the "urban legend" about the "cast-iron 600" slowly became a reality. The breakthrough came when one of my colleagues, who preferred objective evidence over speculation, remembered that he had once picked up a brief reference to the "601 OS" in one issue of the 501 OS parts catalog. The mention concerned only the piston itself with a larger diameter of 85 mm and a clearly convex bottom, but it confirmed the existence of the model and its official name by the factory! As it is easy to calculate, this "600" had a full capacity of 600 ccm, unlike the later known serial 601 OSL engines with a "cheated" capacity of 562 ccm only. It can therefore be considered that the forgotten link between 501 OS/OSL and 601 OSL/OS WH existing in the mid-1930s has been discovered! And not a prototype, but apparently model sold at least in small quantities (note the existence of an oversized piston fi 85.25 in the mentioned catalog, which probably wouldn't bother with a prototype).

The question of the general appearance of the 601 OS motorcycle is still to be answered. Was it, as some say, a typical "500 OS" (i.e. an open bolted frame, a typical fork, etc.) only with an enlarged engine installed? (crankshaft with increased crank and elongated cylinder fi 85) Or maybe "the mysterious bike" just discovered is just in the correct factory assembly? (body "taken" from 500 SS with enlarged 600 cc pushrod engine) The first theory is suggested by the mention of a larger engine in the catalog of a typical 500 OS, not in a separate catalog. Proponents of the latter theory see NSU's desire to create an overhead valve motorcycle well suited to riding with a sidecar (strong closed one-piece frame and high-torque long-stroke engine).

It is possible that the other suspicions are true that this model was never mass-produced and was not officially sold, except for a few prototype or pre-series pieces for trusted customers (e.g. "sidecar" competitors). Perhaps it appeared a little further than another well known prototypes of the "600 model" - the 601 OSB model known from a few photos and one advertising material, company announcement and trade fair report. We invite you to discuss this issue on our forum (link). Or maybe one of you has some unknown documents from the era or other reliable information? We will be grateful for any contribution to solving this very interesting puzzle.

The bike is now restored and running. The owner and restorer decided to keep the existing state of completion and for the time being do not change anything, do not supplement by force.

You can even see and hear how it work beautifully during test drives (link).

The most important thing is that this unique motorcycle is in the hands of a conscious owner and we all hope to see it "live" at the next national NSU rally.