Here is a photo of a tube marked 2040 on the glass and 425A5 on the base http://www.radiomuseum.org/tubes/tube_2040.html , I can't vote for its authenticity but I have both types but not one with both markings.
I have also scraped away the black paint from 1C21s and they have an identical construction inside and work just as well as the others being touch sensitive. I read a few old patents where they mentioned that the black paint was there just there to make them less light and touch sensitive, the other difference I read about was that the ones used in touch buttons were selected for their voltage, which is how some are marked - the last digit in 425A5 marks the voltage 2 = 72V, 5= 75V on the tubes I own. You can read more here as they have been discussed earlier: https://groups.google.com/forum/#!msg/neonixie-l/Ygr3IqWcVDQ/2fCV8ANU5XcJ /Martin -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/c4db918f-2d8e-4966-9575-75ed98c348e8%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
