Reference is made in the bible to 'a prototype RHES arrangement' tested on 5 November 1905, and one assumes the wagon in question was in fact O5 10995 as in the picture above. The brake gear being picked out in white signifies an experimental/prototype purpose. Unlike O5 10793 (bible plate 26 and figure 19) however, which was the 1903 prototype for the subsequent DCI rollout, there seems to be no evidence of the RHES DCIX (figure 20) being adopted at that time on other wagons. What I think is remarkable is that the GWR was prototyping an arrangement long before, but in anticipation of, the BoT 1911 rulings. In the event, the GWR didn't need to get serious about implementing those rulings until the mid- to late-20s, and it seems the DCIX appeared only on V5 minks, so I suspect 10995 probably remained unique amongst the opens.
Well done to Graham and Chris for proving DCIX existed long before it was generally thought to exist!
I'm off to rewrite the gwr.org page (again), and I might feel the need to snaffle a model picture if I may, with your permission of course Dave.