I had got that one in the pic from certified parts corporation years ago, I cant remember the manufacturer name offhand but they had a head temp gauge and a volts or alt meter in the same type housing as well. They had this neat bolt on center bar for the handlebars that they could mount to....this was late 60's-70's stuff. I think Bridgestone/Rockford was the one back in the day selling them as an option and thats why certified has/had them. they are on ebay off an on still, "swapmeetman" usually has them. he has a lot of stock that certified seemed to carry.
Oh yea, they are not very accurate and when you are screaming down the road the tiny screws work loose and the chrome part of the houseing goes flying off into a ditch on the side of the road........or at least that was the case with mine :laugh: