The tach red line would seem like a CB550, either 74 or 75, based on the speedometer number/label style. If you take the tach out of the cup, it should be stamped with a ratio. That will determine if it is 750 or 550. I think the 750 uses 1:4 2500 rpm in, indicates 10000 on the dial. I can't remember offhand what the 550 is. But, I know it is different.