There are two commercial valves out there in this series, the 12Y4 and the 12Y4G. Technically, the 12Y4 should be a metal can style tube and the 12Y4G a shouldered glass style, but in the real world, the 'G' suffix was often omitted by manufacturers of glass tubes. The ARP3 was the military ID for either of the commercial items. They are also referenced as a CV 523. They were designed in great Britain around 1943 by Marconi-Osram. Not sure how that fits into the history of the 52-Set, but this value may very well have been a custom design for that particular set. I have an early 1950 Hickock Tube Tester and it does not show up in the tube charts for it at all.
There is currently a company on the web called Vacuum Tubes Inc that sells the 12Y4 for $4.00 USF. They can be found at:
www.vacuumtubesinc.com
They also show up from time to time on ebay, usually in multiple lots. Typically being flogged by Tube Collectors who cannot figure out what they are used for, so unload them and they seem to go cheap. Lucky us!!
Hope that helps.
David