About the 6061 alloy, I heard from a small one man hydrotest facility that he had found cracks in "some". I don't know how reliable this report was, but it is possible that these were false positives from the original eddy current test device. It was probably 29 years ago, and I don't remember if he said he had actually seen the cracks during a visual inspection, or just during the electronic test.
I did find what appeared to be the beginning of a small crack in the neck of a 6061 SCBA cylinder. I never could determine for sure, but it did show up on the older eddy current test device, and I could see it during a close visual inspection with a magnifying mirror. I discussed this with the owner of the cylinder, and he decided not to take even a tiny chance of a problem, so he scrapped it.
I mention this as a reminder, and hope that no one will get lax on the proper inspections of even the newer alloy type cylinders.
If anyone else has found or heard of similar findings please share the story.