Two contrived Retour covers from Menumbok and Papar

I am gradually building up my collection of Retour covers. These two were recent successful post bids from the SSS Tiverton auction. They are relatively modern but still about 40 years old. These Return To Sender cancellations are largely unrecorded. It appears each town post office has its own version. Previously, I thought that the endorsement was made in the GPO at Kota Kinabalu.  

The sender was hoping that the Mempakul post office was still in operation. It was a well coveted cancellation from the early colonial period. Mempakul still exists but it is only a kampung or village. It is part of the sub district of Menumbok. We can see the uncommon postmark for Menumbok here.

Though not clear, this cover was sent from UK late 1973 judging from the Christmas stamp that was used. The cover languished for about 6 months before it was sent back. The corresponding box hand stamp was applied.

We have a similar situation here with this cover to Papar. It was sent at the same time but took till July the next year before it was returned. There are additional crayon pen markings on this cover. A box hand stamp was also applied. It is evident that this is different in having an extra horizontal line in the middle.
The sender of both these covers was member number 197 of the Sarawak Specialist Society, an expatriate who worked for many years in various parts of British Borneo.