Well, that's very strange that the zoomed pic shows a pretty good "5" at the end of the part number. So, going by the parts book (it's not listed in my March 1966 parts book) I'm at a loss to say what it's for. It looks like a '65 tach but with that part number,
. My experience with 1966 tachs would lead me to the same conclusion: that the GM parts number stamped on the back of the gauge would be the same as what's on the box. I haven't seen them any other way. The tachs in 1966 were made by AC (as most if not all the gauges and senders were) and I would guess that the '65 ones were too. If I find out anything else I'll post it.