RPO hook assembly

Owen:
The diagram is from The Budd RDC in Canada by Ray Corley. It shows a mail hook. This seems to be the only mail hook in the book -- none of the pictures shows one, although some show an iron bar through the mounts.
 
The hook has a small handle on the main bar to rotate it out horizontally.
Some of them seem to have a little circular bit where the angled bar joins on; this is shaped to catch the mailbag.
A supplementary question, Mr. Speaker. Were there any mail routes where RDCs picked up mail on the fly? Were there any routes in Canada where regular trains picked up?