This is scott 492, not really worth much on cover. This stamp seems to have been applied after an initial cancel was applied to the cover then this stamp was applied with a cancel already on it. I also think the stamp was used on another cover because at the bottom of the stamp there is a paper remanent with a little cancel attached to the stamp. You can see the stamp cancel does not match the cancel lines on the cover. when you look under the stamp there are lines that do match the cancel but they only go half way under the stamp and all stop approximately in a straight line. You see a "Y" pattern formed on several of the lines from the back. But on front they are missing. Almost as if another stamp was there at on time. Also the cover was cut open, almost an inch was cut off from that side. What I think happened was that the original stamp was cut into when the cover was opened then someone placed this stamp on in place of it. But why? It is pretty much is a very low value cover.
AAAH!! So this stamp was removed before it was opened. because the cancel lines would match up if they put it back in the proper spot about a half inch to the right. But since there is no envelope left there they moved it over. Yeah even if it might have been Scott 455 not much value either. Thanks Partime!
I see no reason to suspect this was not a full envelope when it passed through the postal system and opened normally by the recipient.
What happened once it got into collector hands starts down the road to all sorts of speculations, some simple (as Occam's razor would suggest) and some sinister and laden with conspiracy theories.
Clearly, the envelope was trimmed significantly at the right side and a stamp placed over the remaining portion of the wavy bars of the machine cancel. Whether it was the same stamp or not is a guess now.
Appreciation of postal history was significantly less at the time of this envelope's mailing. It gives the impression of a beginner in the 1920s making his covers look halfway decent as he cut them down to fit in a small shoebox. Someone doing this would not be at the level of looking for watermarks yet. The value would be based on the stamp, without any regard to the cover portion it now resides on. There might be some nominal "postal history data value" if the machine cancel were an early or late date for this particular machine configuration. I see nothing meant to deceive.