Both the EF91 and the EF80 had American (RMA, RTMA) registrations and American designations.
The EF91 was registered by Cossor as the 6AM6, with registration date 1949 January 28.
Prior to this, did Cossor have its own designation, perhaps in the VPT series, for this valve?
Once the 6AM6 name was available, Brimar switched to this from its own 8D3.
(Brimar had a mixed naming system. For American-origin valve types, it naturally used American designations. For its own designs, it sometime used its own designation style, such as 8D3, 9D6 and 20D4, and sometimes American-style designations, such as 12AH8, 6BW6 and 6BW7. I think that it might also have used Pro-Electron designations as primary names to some extent in the 1950s.)
The EF80 was registered by Philips as the 6BX6, with registration date 1951 February 15.
It would appear that at the time, Philips was routinely undertaking American registration of its new noval valves. In part, this was a requirement for the Australian market, which generally used American designations, as noted in the “Innoval” thread, at:
https://www.vintage-radio.net/forum/...d.php?t=159120.
Cheers,