Out of curiosity, I did a search on booster-retarders and found this interesting reference, it is about Welsh Mining and an interesting read (search document for 'booster-retarder'). It mentions Dowty, the manufacturer, I visited their manufacturing plants in the '80s. Their main market for these was in South Africa, they had a manufacturing plant near Johannesburg.
http://www.davel.f2s.com/hendrecoed/...sletter/54.pdf