Based on my research so far, I have come to the conclusion that one must divide the use of these "Clerk Daters" into two areas.
First, the documents, forms, packing slips, etc. that had to be filled out. The 1887 Regulations state that the RPO Clerks were allowed or supposed to use these private stamps for this purpose.
Second are the mandatory backstamps on the Registry and Special Delivery letters. These were, according to "Postal Buletin February 23, 1924", to be backstamped with the "receiving stamp of the office" and not the private stamps.
That the private postmarks were also used there cannot be denied because of the available covers.
The private stamps must have come into use very early. This is proven by a letter with advertising for these stamps, of which I can no longer trace the origin. In any case, it does not come from this threat. I show it here.
This is a summary of what I have been able to get my hands on so far.
Discussion and criticism definitely welcome.
P.S.: this "work" of researching a subject or a letter is what I like about this hobby. It is always a challenge.