What Kind Of Rpo Cancel Is This???

No "provisional" use would EVER have been widely tolerated on registered mail. You are mis-reading the regulations to get any interpretation that these devices were contrary to regulations.

They are specifically mentioned in my post of July 20, 2020 as "rubber road stamps", thus they are accepted, etc., regardless of who paid for them.

You may have to do the original research in the primary documents yourself. I do not have a citation to satisfy your demand for a formal reference beyond what I have posted. Good luck.

I found this:

"Regulations from 1887

CHAPTER THIRTY-EIGHT.

THE HAHDLING OF REGISTERED MATTER BY RAIL-WAY POSTAL CLERKS.

Sec. 1225. Postmarking and Returning Registered-Package Receipts.—To protect themselves against fraud in the matter of receipts given to postmasters and others for registered matter, railway postal clerks may affix the imprint of their postmarking stamps on each receipt as many times as there may be packages, and return the package receipts by next mail to the sending postmaster or employe* of the Railway Mail Service. (See section 1068.) If a registered package should be received by a postal clerk without an accompanying registered-package receipt, he shall fill out a receipt for it and send it to the postmaster from whom he received the package, and report the fact to his division superintendent."

This may be a clue. As I understand it, there is reference on the Clerk's "own" stamp (marked in red).

My search for the words "rubber" and "dater" were unsuccessful.

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.

Juergen

P.S.: this "work" of researching a subject or a letter is what I like about this hobby. It is always a challenge.