Far East Driver
I'm a bit confused by your reference to DC (B) - did you mean Direct Entry Commission?
When I applied to join in 1965 the DEC ( B ) was offered, as were DEC ( A ) and ( C ). A meant a commitment to 38/16 point, B allowed the individual an option to leave at either 8 or 12 years from joining and C offered a further option to leave after only 5 years. The option was the individual's and not the service.
I did hear the occasional tale that co-pilots on the Herc' would not get a command unless they waived their 8 year option well before the period stipulated in their original choice.