My, my, my......What Do We Have Here?

Sorry to hear of Your purchase that went sour! You might want to remove the faux chrome covers off both tach and speedo to compare paint quality on black portion of instruments? A great indication of being in an equal environment for all those years. Also the gauge face color should be similar. Look for tool teeth marks on speedo cable end too. Doubtful the speedo was "turned-back", probably just replaced. Speedos cannot be turned back on these style instruments, unless they are taken apart to access the register clip. Of course they can be reverse ran for hours in a drill of some sort to go past the limit of register.

A parked motorcycle is just an ornament, until it builds a fire

AMEN

That is why you should never park a motorcycle during its storage...