I can't offer any specifics but hopefully can answer a couple of points.
This transmitter covers the HF bands, it would have been most likely been used for commercial shipping communication.
I'd date it a little later than RM does, I'd say mid to late 50's.
The reference to "RF truck" means the unit was mounted on wheels, and could be rolled and repaired or replaced in the event of failure. It would never have been lorry mounted for mobile operation.
STC could never compete with Marconi Wireless in this country, most of their kit went for export to Europe and Africa. A lot was manufactured under license in Australia.