You wrote "A nice story for someone to uncover here!" Because I'm always on the lookout for unrecorded Canadian Military postmarks, I did some uncovering to check the "No Record/ GHQ 2 ECH 21 Army Gp." hand stamp for Canadian-ness.

During World War II, members of the Citizen Military Forces (CMF) fought overseas in New Guinea and the South-West Pacific Zone. The CMF was a volunteer militia that was originally tasked with defending Australia. However, as the war progressed, the CMF was required to serve in other areas.......

I'm pretty sure that the 'No Record/ GHQ 2 ECH 21 Army Gp.' handstamp is a New Zealand unit mark. The big circle Canadian mark is Sayles F403, applied in the UK. Two days later the letter was in the hands of the NZ military PO in the UK.