We all know what they look like from the front. Here's a couple of pictures showing the back of the stock gauges. From left to right: clock, outside temperature, oil temperature.

Here's a picture showing a standard VDO Contour voltmeter (on the left) and the oil temperature gauge from my car (on the right). The gauge from my car is deeper, presumably to incorporate the special connector and conceal the internal wiring. All of the gauges from my car are the same depth. The standard VDO Contour and Cockpit gauges are both of the same shallower depth.

One other difference of note... there are small extrusions at the top and bottom of the body of the gagues from my car, which fit into notches in the holes in the center console. these help you keep the gauge straight up when installing, and prevent them from spinning after installation. On the right side of the gauge from my car, you can see the small extrusion near the bezel. There's one on the opposite side of this one, it's just not visible in the picture.