Thanks! And yes that's the Mercedes 250c in the background. I would definitely describe it as a tank. A tank in evening dress perhaps. It was very elegant in silver with a dark red interior. Here's a better shot of it.
I enjoyed driving it a lot but the twin Zenith carburetors on the inline six were always problematic and after the second carb rebuild (not a cheap project) I gave up and sold it to a guy in Florida. I did fantasize briefly about dropping in a fuel injected motor from the 280sl. That would have been a neat project but sanity prevailed!
I never had a 550 but by all accounts I think it was a better bike for the US market than the 400F. But the 400F was always my favorite. There is a story to that as well. When I was at the University of Maryland in the late 80s there was a woman who rode a red CB400F with café bars. She wore a black leather jacket and a black helmet and I thought she was the coolest person on earth. At the time I had just bought a ratty Honda CB500T as my first bike, which is possibly the worst motorcycle Honda ever made. And mine was possibly the worst ever example of the worst ever Honda. Bad electric blue paint, nearly bald tires, rust holes in the exhaust... and the timing chain tensioner was utterly shot so it rattled like a coffee can full of bolts. In comparison that red 400F was the most exotic machine I could imagine. Early imprinting is everything and eventually I went on to own several 400s, in various conditions from flat-black-rat-bike to immaculate original. They are lovely bikes if you don't care over much about power.
I think I've owned something like thirty or forty motorcycles in my life, some great some terrible, but there are a few that stand out. The CB400F is right at the top of that list, along with the Moto Guzzi Le Mans, BMW K75s, Yamaha RZ350 (in Kenny Roberts yellow and black of course) and the Yamaha SRX600 - which is a hugely underrated machine. I could tell so many car and motorcycle stories. How I came to own a 1966 Maserati Sebring for pennies (relatively speaking), riding to Eastern Washington in the middle of an apocalyptic wild fire to race mini bikes on a kart track, the formative experience of riding in my grandfather's Maserati 200si racing car as a boy, the challenges of racing a Honda 125GP bike on a credit card budget, and many more... but I suspect that they would be better fodder for the bilge, which is a place I don't go very often.