The mysteries of a Brunei Local

The Brunei Locals refers to the issue of 1895 stamps using a "Star and Crescent" design. They was initially thought to be bogus but subsequently, were proven to be genuine. They were issued after an arrangement between the then Sultan and a J C Robertson to fulfill a local postal purpose. The main motive behind it was to raise income for both parties. They were only valid within Brunei and for mail to Labuan as Brunei was not a member of the Universal Postal Union.
The initial impression was that this could be an rare and unusual stamp with the date being written to replace the faint underlying postmark. On closer examination, one can see remnants of the postmark with the date of 4 AUG 1895. It is unusual that the writing and the underlying postmark seems to be made with the same ink. The postmark also  corresponds quite closely to D1 of the Proud classification.
The date itself is a problem, being a Sunday! The other Sunday dates recorded on this set of stamps include 17 MAR 1895, 18 AUG 1895, 27 SEP 1896 and 23 JAN 1898. A whole set with SON(socked on the nose) or bullseye postmarks dating from 20 FEB 1898.  So are these bogus postmarks?
Bogus postmarks tend to have fixed dates as they normally do not to have interchangeable date slugs. Secondly, why  chose a date which corresponds to a Sunday on purpose (or through error at least six times!)? The postmark is almost certainly D1.
There is a possibility that Sundays were normal working days in Brunei at that time. Being Muslim, the day of rest would have been a Friday. Or quite possibly, the post office was open 7 days a week.
The most likely explanation was these were cancels by favour. One can imagine that Mr Robertson found himself free on a Sunday. He knocked on the door of his friend, the postmaster, who helpfully CTO stamps, perfectly SON, that were requested by collectors outside of Brunei.  Some of these stamps happened to have faint postmarks and they thought it would have been a good idea to insert the date by hand using the ink from the same pad.
For more information on this fascinating issue of stamps, may I refer you to the book "The secret lives of the Brunei Locals" by Brian Cave.