Tri-X, Plus X, Super XX etc all came out in 1938/9 as a complete new family of films. I've posted the data page from the 1940 Kodak Ltd Professional Catalogue here on APUG in the past (not sure where). In fact they were coated in at least 3 facilities at that time, Rochester (Eastman Kodak), and Kodak Ltd's UK Harrow facory and their Hungarian plant, possibly at the Candian plant as well.
So it was rather a shock to see Kodak celebrating 50 years of Tri-X many years too late
In fact it wasn't initially sold as a 35mm emulsion probably becuase earlier versions were felt to be to grainy for the format.
Plus-X Panchromatic, Super-XX Panchromatic and Tri-X Panchromatic are the original names, I'm looking at a 1940's Dataguide. They were released after Iflord had upgraded their films with the first generation of Fine Grain Panchromatic and Hypersensitive Panchromatice which became FP2, FP3 then FP4 and HP2, HP3, HP4 and now HP5. Tri-X has been through as many revisions over the years.
Tri-X was 200 ASA Daylight and 160 ASA Tungsten light, for sheet film Kodak recommended DK60a, 6 mins intermittent agitation. However when the speed changed it was down to a change in the ASA (also BS) testing which removed a safety factor of a stop, the emulsions themseves didn't change and this affected every manufacturers films and ASA/BS rating
It's possible the research etc for these films was done in the UK there does appear to have been a wider range available here in 1939/40 than is shown in US publications.
Incidentially the Fortepan 400 was derived from Tri--X, the Fortepan 200 from Supper-XX, both were made in Kodak's Hungarian factory which later became Forte.
Ian