The North British interlaced turnouts could have lasted well into the Grouping and BR days. Apparently there was a habit of continuing on old practices after the Grouping, certainly in the Caledonian and probably the North British as well. I have a photograph of an interlaced tandem on the Stobcross - Maryhill line in Glasgow dated 1955 still with Caledonian chairs. I also have a picture (somewhere!!!) of an interlaced turnout on a loop on the Perth - Aberdeen line taken in the 1970s where the chairs are either LMS or BR and the timbering looks much younger than fifty years old.
Jim.