Way Pocket Or Wp Auxiliary Markings

I am seeking information on the auxiliary markings "Way Pocket" or "WP" on 1880s covers like my covers that are shown above.

My current understanding is that the 'way pocket' was originally the front left pouch on the saddle of Pony Express riders in the 1860-61 time period and that it held the waybill and some mail. After 1861, stagecoaches also used a 'way pocket' pouch for similar reasons. The US Post Office Department used the term "way pouch" or "way pocket" to refer to a pouch that contained mail meant for Post Offices along a certain route (as opposed to the end destination PO of the route). The "way pocket" or "way pouch" would be opened at each office to remove local mail and/or to add mail from that office to other destinations on the route.

My questions are; why do only some covers carry the "way pocket" auxiliary marking? Why bother to add the auxiliary marking rather than just putting it into the way pocket/pouch?

In terms of context, here is what I have learned so far. The term way pocket seems to have begun in 1860 with the Pony Express; it referred to the front left pocket in the saddle and contained both the way bill (filled out at each station) and way mail. Of course the original Pony Express only lasted a year or two (coast-to-coast telegraph pretty much ended finished it off) but the way pocket or way pouch contained on for many years with Stage Coaches and in other uses.

My covers are from the Montana Territory and at this time it was truly 'wild west'. Postal stage coach routes were rife with fraud and political corruption. The addressee of these cover was a significant influential business man. He controlled much of the business in Virginia City MT which was a mining town of around 10,000 in the late 1880s. His store was the very first brick building in town and used as a test to see if they could produce and use local bricks. The building still stands today.

Below are some related images.



1875 store


1878 store


2020 store


Montana Territory Stage Coach circa 1878


Stage Coach Way Pocket

Don

Great thread! enjoyed that.
Wooden shutters there, to cover the windowed alcoves 1878.
Looks like the original mounting points still there in 2020. Amazing!