As a person who has a PO Box and who has sent and received a number of CMRRR letters, let me answer this:
When a CMRRR letter is addressed to a post office box, the mail sorter places it in a designated spot behind the counter at the post office, and he places a yellow card in the post office box.
When someone comes to collect the mail from the box, they find the yellow card which says "Take this to the counter to pick up your mail." It has a series of check boxes on it to indicate if the object was not left in the box because it is too big, because a signature was required, etc. (But in my experience the clerks never check any of these boxes.)
The person takes the yellow card to the counter, the postal clerk goes someplace out of sight for a couple of minutes and returns with the CMRRR letter. The customer then signs the card and takes the letter.
