T or Unpaid mark on North Borneo covers

I have been looking at T marks recently. This is partly due to the recent appearance of a 1889 North Borneo combination cover from Kudat with an unrecorded T mark in a major auction. According to Proud, the earliest T mark appeared in Sandakan in 1891. The UPU decided that mail with not enough postage should be marked with a T  from 1 April 1879. The amount missing would also be indicated in black. By convention this amount was expressed in UPU centimes tied to the value of gold. The earliest postage due stamps were issued by France in 1859.
The amount charged was usually double what was missing to include a handling charge. With the varying value of gold, the exchange rate calculated became quite difficult. For example prior to 1940, one GB old penny equated to 10 UPU gold centimes but this went down as pound sterling depreciated postwar. Nowadays the Royal Mail charges a flat fee which includes handling.

This cover to the rare destination of Algiers was underpaid by 6c . It was sent from Sandakan on 4 JUN 1932. It has the T mark UP4(1898-1941). It is also a Retour cover with various return instructions. No payment of postage due was indicated and the cover was supposed to be redirected to New South Wales in Australia. One would have expected to see an Australian arrival cancellation.

This is a much later cover in 1955 with UP7(1947-1956) sent from Sandakan to Kampar in Perak. This is from a group of almost identical philatelic covers sent to J Bailey. It has a Malayan Postal Union 8c postage due. It has the correct rate of 4c for printed matter but was treated as ordinary mail possibly because the back of the envelope was sealed. Early postage due stamps from North Borneo were often cancelled on a payment memo slip separate from the cover. Such items from North Borneo can be expensive.

The Tenom UP4 mark is identical to UP7 from Sandakan. It is probably very uncommon as Proud only gave one date of 18.9.57 and this one here is from 3 NOV 48.