Well I don't know how it worked for the jets but (circa 1960s) for the Fokkers a port "controlled" a flight. For example if it was a western Qld flight which called at Roma , Roma controlled it; if it didn't Charleville would control the flight. If it extended to Longreach, Longreach would. In the applicable TAA office was an A4 sheet of cardboard with the numbered seats on it with each port on the flight allocated a certain number of seats If someone rang or came into the office and wanted a seat, their name would go on the seat on the card if the port allocation wasn't exceeded. If it was, the agent would then ring or more likely teletype the controlling port to release a seat.
If say someone in Sydney wanted a seat for a passenger Longreach to Charleville, they would telex Longreach (the controlling port) with pax name with an "NN" meaning need a seat. The agent in LRE would check the card and then acquire the seat.and then send a return telex to "HK" ie confirm the seat. If the flight was full the telex would read "HN" meaning waitlisted. (I think the codes are correct. It was along time ago.)
The actual weight and balance and the load planning in terms of what goes where was done by the first officer on a sector by sector basis.