Correct, flange, I should have clarified that.
Few H vans had vacuum bags - I think is as one on a single occasion, though (a) I could be wrong, and (b) it might have been a one-off for some specific purpose.
For a modeller, all grey is the rule prior to 1970, bubbles included. There were extremely few exceptions, though one which springs to mind was the short lived red on Ranks grain wagons, which were hardly commonplace! Post 1970, brown graduates from start to about 60% (maybe 70%?) of working stock by 1976. From then on, it's virtually all bulk containers, bogie fert etc. All brown.