Nice shot! you caught it right after the bag snatch.
Operational comments: The bag holder (land side) was rotatable. The mail pouch was installed on it while it was turned sideways to the track so it was out of the way of any other trains. When the mail train approached it was swung into position.
The upper arm was fixed but the lower arm was a weight to hold the pouch straight. When the bag was snatched the lower arm gave enough to release it and then dropped away. I'm not sure if it slid up and down the post.
I'm also not sure if the posts were always on the same side of the track. I think that the RPO section was always run in the forward position in a baggage/RPO car (for security, you know). edit: ( just looked at the photo again. Apparatus on the left and RPO section trailing the train.)
There's a lovely British film (Night Mail ?) showing inside train operations. There's a sequence of them hanging the bags out of the TPO (Travelling Post Office) where the supervisor tells the clerk "Now after we pass X, three overbridgesthe count 17 seconds." You hear 3 swooshes, then the count, the apparatus is swung out and WHANG!