With a magnifying glass are there any numbers, letters codes ect. More pix of the entire tach would help.
Looks like an old Motorola tach...can you pull the back off...usually there are numbers on the inside case. I just picked one up at a swap meet for cheap and found out it was for a 6 cyl and had an hour clock on it. If I put it on my V-8 it will read 3/4 of the RPMs according to Prestolite which bought the tach section of Motorola years ago.