Triotron was the brand name for tubes manufactured by Radiowerk E. Schrack, Vienna, starting in about 1926. However, in 1939, Philips acquired control of Schrack and Triotron and changed the name to Wiener Radio Works (WIRAG).
Google is pretty sketchy, but it appears that Philips continued to use the Triotron brand in Britain, but only as a distributor, not a manufacturer. So there is no way to know for sure who manufactured these later Triotron tubes, but since the brand was owned by Philips, I think it is likely that those EL11 were manufactured by a Philips subsidiary. But until you have them in your hands and examine them closely, you won't know for sure.