I have a mint set,possibly even unissued of 68 Pattern DPM Smock and the even harder to find Trousers in a Size 9 going spare.
Size 9 by the way was the largest size made(other than special orders)
Funny enough,and not much help in your search Im afraid but I also had 2 pairs of blue overalls/boiler suits when I did YTS in 1987 and from memory they had rubber buttons.