Or you can count them while they are all together...
The housings on the top are from 911 engines, circa 1963 to circa 1966. That's right, 911 engines, NOT 906!
So when you see them for sale and the ad states: "These are chain housings for the 906 & 910" and the picture looks like the housings on the top, they ain't 906 or 910! First of all, the 911 housings are aluminum and the racing 906 & 910 are magnesium (we'll talk about the 911R later). Back to, can you count the studs? The 906 chain housing cover will not work on a street engine and vice versa so don't let these guys (whoever they are) convince you that they will! Look at the lip on the aluminum housings...that lip is to contain the perimeter sheet metal like all street engines have. The 906 & 910 don't have sheet metal around the engine. Okay, they don't have fiberglass around the engine (because the racing motors have fiberglass instead of sheet metal parts).
Hope these "Did you knows?" help educate the uneducated!
I forgot to tell you, the threaded bosses in the front of the covers are for the oil feed for the rifle drilled camshaft. The oil is dispersed through the middle of the camshaft and empties on every cam load to lubricate the surface between the cam and the rocker.