The great thing about philately is there is always something new to learn. Recently, I bought a North Borneo cover with a black square diamond mark on the front. I am grateful to the vendor who is very knowledgeable and he directed me to where I can find further information.
This cover was sent by Harrisons & Crosfield to GB on 10 OC 63. At independence on 31 August 1963, the name North Borneo was changed to Sabah and it became part of Malaysia on the following 16 September. So this is an early cover from Sabah as part of Malaysia before Sabah overprints and postal handstamps were issued.
Now, back to the census mark. This is usually in black and relatively rare in red. This was applied at a GB post office when a census of all mail passing through was carried out. This was normally in the month of October and sometimes in November. This is for both internal and external mail from 1922 to 1985. It was used after 1985 but not for census purposes. The following image is taken from the Stampboard thread which explains it very well.
This should be an uncommon one from Field Post Office 766 Labuan. The date is not clear but is probably 12 OC 1965
References:
https://www.stampboards.com/viewtopic.php?f=9&t=50015
http://www.vicstamps.com/displays/postal_census/postal_census.html
