Actually not that old. BR Standard stock was not introduced until 1951, until then the former company designs continued to be built, in their traditional workshops, and a lot of what was categorised as "pre-nationalisation" stock at the end was from then. Wartime lack of maintenance plus absence of any new build meant a lot needed replacing. Meanwhile the BR maroon livery gave way to blue/grey in 1965, so there was only a 14-year gap between the two, although not a lot of pre-standard stock was actually repainted.Not the oldest but some LMS 6 wheeled bogie sleepers lasted into the 1970's and received blue / Grey
Those LMS-design 12-wheel sleepers were actually built in 1950-52.