I couldn't find any pictures, but I don't think you have to worry, there box was a full range box about 45Hx30Wx24D. Malcolm Hill retired in 1992 but his son is running rock-tech the links at the bottom. Contact him he should have plenty on the old systems. But here's what I found out on the web so far. From some web-site:
The sound contractor for this festival was Malcolm Hill. Hill Audio was a well-known native company based in Hollingbourne, U.K. In the great Tradition of the Times the company was named after, owned and run by Malcolm Hill. Malcolm and his employees designed and purpose-built a great deal of the equipment in his own shop. Speaker cabinets, power amplifiers and mixing consoles were all custom made and proprietary.
From Prosoundweb:
I guess it works with the Hill PA system(3 12", 2 10" and 1 2" motor on a 60x90 horn and the sub was another 12" in a short box that has the same footprint as the HI/Mid box.