[MrFixr55]

I've seen companies, especially smaller brands, but also Emerson, RCA and others either use up stock before switching to the latest and greatest tube that does the same thing (Such as switching from 12K7 to 12SK7 to from 12SK7 to 12BA6, etc.) or buying the older tube at discount and only switching when they were not available. I have one RCA Model having only Octal tubes when introduced, then both Octal and Miniature tubes, and finally Miniature tubes. In one set (can't remember the brand, It had 12SA7, 12SK7, 12SQ7, 35W4 and instead of a 50L6, it had a 50B5. In another set, they were all miniature tubes, but the holes in the chassis were sized to take octal tube sockets. There were companies that made miniature sockets sized to fill holes drilled for octal sockets.