thought I would post the complete history of the 4.3 liter that I could find. These are links and articles that I have bookmarked for my own use, however they should help many people. it may help some people when looking for parts or sourcing a salvage yard motor or a reman. keep in mind that...
forums.iboats.com
Looking through your post,
you're not using part numbers for the iron or steel pieces. Those are casting numbers. They're not the same thing.
A part number can also have a cast number, which is different. The factory that created the metal block, cam, crank, or whatever metal piece assigns a casting number to it. Then, it gets sent out to other factories in the supply chain of different companies to use in their final product.
Forge/Factory ABC will create engine block 123 because they have the resources to do so, along with economies of scale - giving it the competitive price for other companies to use in their assembly. Then ABC will send the engine core with it's cast number on it to companies XYZ and EFG to use and/or modify it as they see fit for their product and then assign their product number to it.
So, the factory/company that made the metal item will put it's cast number stamp on it, and then the customer-facing company that ends up using it in their final product assigns it a part number.