So, let's move on. While disassembling the individual bodywork and mechanical components, I noticed that the frame was already in the traditionally "aluminum head" version, meaning it had a cylinder head with covered valves. It was supposed to be made in 1937 because the forging under the seat had the date 12.37 and an additional number, which turned out to be the number of the main frame pairing with the rear triangle (surprisingly the wide type for fully enclosed chain guard?!). Was everything from the so-called "single source?" A query to Audi Tradition further confused the matter, as the production date in the NSU archives was February 1938. This information somewhat undermined my knowledge of NSU, and I began to more thoroughly analyze the catalogs available to me. After all, NSU has some of the best records of changes made. And I found something that, ironically, is the bane of all discussions with NSU motorcycle owners. When you point out that a given component or assembly was used in a different year or model, the magic phrase "it's probably an adaptation" is uttered, explaining all the mistakes in assembly and model identification by the owner. Well, it certainly sounds better than a "folding" or "mongrel." And here comes a true "TRANSITION". A model (or rather series) combining the features of exposed iron "safety pins" cylinder head with the later version of the motorcycle with an aluminum cylinder head. In the 351/501 OSL parts catalog, it's described as "with a cast iron cylinder head and closed chain guard," with frame and engine numbers ranging from 1065220/310331 to 1067472/310527 (this example has a number indicating it's one of the last in this series). The problem with NSU numbering is that it's impossible to determine the number of units produced for a given model based on the frame numbers, and it's difficult to determine how many "transition" units were actually produced. The difference between the frame numbers is 2252, meaning that during this period, NSU produced exactly that number of motorcycles across all models. But the difference between the engine numbers is only 196! Is this the actual production number of this transitional model, where the main difference is the engine design? I haven't been able to determine this, but I still have the impression that these OSLs are the shortest cataloged series in NSU history.

Once we figured out what it was, things got tricky. The engine was original, but it was "mismatched" and cracked. Someone had fitted it with a smaller-bore cylinder from an older, long-stroke 350. If it had been a 500, there wouldn't have been a problem, as the 501 OSL's wide-block engine could be fitted with a cylinder-head assembly after replacing the studs. With the 351 OSL, the piston stroke and cylinder bore had been changed, and nothing from the older version would fit. Fortunately, the bodywork was relatively complete, and the missing parts could be replaced with new parts. The search for a cylinder and the mechanical parts took quite a while; there were even suggestions for a temporary replacement, but thanks to the help of colleagues on the NSU-Riders.pl forum, we managed to assemble the correct kit. Then, everything was overhauled. Starting with the mechanics, proceeding through the bodywork, the electrics, and the carburetor, all the way to the final result: a running motorcycle.

The renovation was completed in 2022, and since then Krzysztof has been making his presence felt at vintage motorcycle events, including NSU rallies.

A brief summary of the differences between the 351 OSL models from the turn of 1937/38:

(The above information is provided based on my current knowledge. If anyone has verified information that is inconsistent with this text, I will be happy to verify it.)

Radek Mikulski