BOAC bought a 707-379C G-AWHU off the shelf (already painted and built for Saturn but their order for 3 was NTU) to urgently replace the lost G-ARWE,
Likewise G-AYLT was the last 707 bought by BOAC to replace the SVC10 lost at Dawson's Field, Jordan.
As WHBM outlined >
The 707-320B has the range benefit over the -320C but many Legacy airlines bought both types.
The -320C at the time was chosen early on by a lot of airlines to also enable MAC work (US Troop movement charters)
The Vietnam War was in full swing - However the trend changed to ordering the DC-8 63CF series especially for US supplemental charter airlines.
Some airlines cancelled their 707C orders to buy the superior DC-8.
Trek Airways had ordered a new 707-350C from Boeing for its JNB-LUX route in association with Luxair, but the aircraft went from the production line to SAA to replace their brand new 707-344C 'Pretoria' lost at WDH in 1968.