Chevy Big Block Casting numbers

[joewill]

I currently own a 1970 3963512 454 Block. I have been watching Chevy Big blocks for sale and I see certain casting dates advertised. Some of these blocks have next (under) to the 3963512 casting casting numbers a 'T' or a 'P' or otherwize nothing. What do these mean? I have been trying to find a correct block replacement for my corvette based on date code but 2 blocks close in date may have a different T or P or just blank. I have heard T for tonawanda, and T for truck, and P for passenger.. does anybody know what these mean? I have seen 4 bolts have one and 2 bolts another and it seems to be random.