I've sonic tesed over a dozen 400 blocks, no good patterns on year or castings dates I've seen
Buy the blocks on a condition of passing your sonic tests, if the seller will agree with that go for it It will cost you for the soic testing(unless you buy the tester and and start offering your services to customers ) but nothing good comes cheap, ever. I bought a Dakota 1000 years ago, still have it and use it now and then as needed