1967 400 block with 68 castings...has anyone come across these before?

I bought a 1967 gs400 4-speed convertible 2 years ago and was told it had a 68 400 block in there, as seen by the block casting number of 1383790. It also has intake casting of 1383448-1, which is found for a 68 Electra or LeSabre with date of June 24. Finally, the engine was pulled and has head casting numbers of 1368985 , which decodes to 68 430 block, with date codes of May 08 and June 20. But to my pleasant surprise, the engine shows an NR code and most importantly, matching numbers to my trim tag. The transmission, which is original, shows a production date of June 21, 1967, and my trim tag says 06D, for production in the 4th week of June. So the theory that was shared with me is that the 67s were notorious for cracking around the valve spring seats and that Buick may have seen enough of this by the time the car was being built in June, which was late for a 67 model, that they put some second generation parts on it. I can't find any literature to confirm this but was wondering if anyone else had come across something similar.