I was at a machine shop a few month ago, and blocks were stacked every where. I found a 400 block that was a bit odd ball. It has 2 bolt caps on it, but was tapped for 4 bolt caps. So.. I bought it. Casting number is 9799915. Looks like this is from a 70 pontiac. I was now thoroughly confused. I didn't really care, I have a 4 bolt main block on a great casting. One of my buddies who worked at GM for 25 years was pretty intrigued by this as well. He sent me this:

The PN 9792506 was carried over into 1968 for Ram Air applications, while Pontiac created the PN 9790071 casting for two-bolt applications. By year's end, Pontiac determined that two separate castings were unnecessary and it began drilling and tapping the PN 9790071 casting to accept four-bolt main caps as required for Ram Air applications. The PN 9790071 was carried over for 1969, also using the same practice of for attaining two- and four-bolt blocks.
The 1970 model year saw the introduction of yet another 400 ci casting. It differed in that it was equipped with all of the appropriate mounting bosses to fit the new-for-1970 Firebird chassis. Despite being drilled and tapped for four-bolt caps, Pontiac installed two-bolt caps on PN 9799914 for typical applications, while a second casting, PN 9799915, was limited to four-bolt Ram Air applications. The PN 9799915 was actually created by grinding away the "4" of the "9799914" part number and hand-stamping a "5" into place. The need for Pontiac to require two identically machined blocks remains unclear, and in rare instances we find where Pontiac used the PN 9799914 or PN 9799915 interchangeably.

I thought this was a cool piece of info, and wanted to share.