I finally confirmed that it is a CB500 fuel tank. It may also be compatible with the CB550, but that I don't know because my CB550 is a later model bike ('77 CB550F) which has a noticably different tank. If the CB500 and early model (or K model) CB550 shared the same tank, then this tank will fit both the CB500 and CB550.
No dents. Honda emblems are attached, fuel petcock included but it is missing the fuel cap. Previously, I mentioned this tank was clean inside. I was going by memory. In fact, it does have some surface rust and would probably need to be cleaned up and maybe sealed before use.
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I have another CB500 (CB550?) tank as well, which can be seen in this picture (mounted on the bike against the fence). It is the same as below (no dents, some rust inside, missing fuel cap), but it is primer grey. The Honda emblems are not attached, but I have them to provide with the tank.