I went to WINEPEX put on by the Redwood Empire Collector's Club on October 6-8, 2023. There I found a various interesting covers one of which prompted me to post these photos.
There was no USPOD minimum size rule for envelopes for decades, well over a decade. While there were requests to use envelopes of a size which could be easily handled, the USPOD lead the way for small envelopes (1-3/8 x 2 ¾) - Size #1 [note (No. 1)" in the left corner card] envelopes which were smaller than what customers were being asked to use:

After decades of pleading and the rise of high speed cancellation machines, an minimum size was adopted as noted here by a too small of an envelope. Over note, this too small envelope was returned to the sender inside a proper sized envelope (see manuscript notation).

I included this small one as it is of more interest from Woodbridge to Dixon within California than just the small size.

Next I found a rather small mourning cover, well smaller that normally found. Just a few covers away I found the second one below to establish a very small mourning cover.

This mourning cover is about 1/2 centimeter shorter than the one previously shown on page 8. It is not as tall as well.

Now the cover for which I was motivated to post, both the from and back, pick which is which as the stamp is on both and is also cancelled front and back. Of course, "the front" is the side with the address.
Front-

Back-
