I realize this is a very old thread, but there is a difference betweent the 7708 and the 7712. GM didn't use different id numbers just to confuse us. If there is a different number, there is usually a difference, even if they are interchangeable.
They both start out from the same casting, as noted above. Timing marks are in the same place, and yes, they would interchange.
The machining is a little different. The 7708 has a lip machined into the back side from the outside in, making most of the outer ring about 1/16 inch thinner. The machining starts just outside the elastomer (sp?). It would be very easy to make a 7708 out of a 7712 with a lathe.
Can post a pic of them side by side if anyone wants to see the difference.