This is from the site Bring a Trailer (https://bringatrailer.com/) ...
This 1951 Nimbus Model C is an interesting Danish bike utilizing an OHC, air-cooled inline-four and shaft drive, a layout with roots traceable directly back to their similarly configured 1919 model, the seminal “Stovepipe.” It’s also worth noting that Nimbus was the first to offer a telescoping front fork in 1934, a year before BMW. This one looks to be a military example, and is said to have spent decades in a museum. Regardless it’s now reported to start easily and run well through all gears, though the clutch is slipping and will need to be adjusted or replaced. Find it here on eBay in Scottsdale, Arizona with reserve not met.
Reads the ad: “You are bidding on a true survivor, A 1951 Nimbus with sidecar. Nimbus was a Danish-made motorcycle with an inline four-cylinder with external rockers and a stovepipe exhaust. This bike was in a museum since the early 60’s with true mileage. The bike starts on the first kick, runs through all the gears, and brakes well, but the clutch is starting to slip. I take the bike for a ride almost every other week to keep her legs stretched. The original title was lost years ago, but I can get an Arizona title if you wish. I’ve had the bike for about two years.”
The engine is a 749cc unit with an unusual overhead cam/exposed rocker setup seen in detail here. This one was probably set up for sidecar duty from new, with a 3-speed gearbox allowing a top speed of around 55 MPH or so.
Check out that key–very cool. Mileage is give as just under 10k, which corresponds with what’s recorded on the Smiths combination metric speedo/odo unit–call it 6k miles.