OEM Tach Expert Required

[Nanook]

I recently purchased (separately) two factory tachs, but as you can see in the pictures they are quite a bit different looking.

The one with the red line tabs is supposed to be for an eight cylinder and the other for a six.

The six-cylinder tach is connected differently from the eight-cylinder in that the + side is connected to the coil and the other terminal to ground, whereas the eight-cylinder is connected with the + to 12v and the other stud to the coil with the third terminal being grounded through the case.

Has anybody else come across something like this? Thanks