This '74-'78 Honda CB550K, sporting a luggage rack and aftermarket exhaust, is a classic UJM, for "Universal Japanese Motorcycle".
Nowadays, bikes are sold from the factory as tourers or sportbikes or cruisers or whatever, but that wasn't always the case.
The CB550 came in two forms: The CB550K "Custom", which had four-into-four exhausts (the four separate trumpet-shaped mufflers were quite dramatic looking) and the CB550F Sport, which had lower bars, a differently-shaped, chromeless gas tank and a factory 4-into-1 header.
From this basic platform you essentially built the kind of bike you needed. Need a commuter? Bolt on a luggage rack. Want to go touring? Add a Vetter fairing, saddlebags, and maybe a trunk. Feeling sporty? Rearsets, flatter bars (or maybe clip-ons), aftermarket racing exhaust, and maybe a little bikini fairing.
Pretty soon the factories began offering bikes with all these features from the factory, and the basic motorcycle became a lot less common, although they get revived out of nostalgia every so often.
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