I tried out a bunch of 12a...7 tubes and their ilk that I bought in a lot from eBay. Most of them turned out to be too microphonic and unuseable, but one tube that that sounded great is simply labeled ‘5963’ with no other markings, (as pictured). Can anyone give me more info on the identity of these tubes and when/where they came from? I’d like to know/buy more. I think the auction said they were RCA brand tubes.