Scott 405 1c Washington oddity, any ideas?

Hello Rod, rod222 Thank you for the image, I finally have an image of my #405, but for some reason can't upload it, maybe because I'm a new member? Your image says it all. It's just that I had never seen anything like it before. Thank you for taking the time to explain things to an old newcomer.

It's actually a little bit more involved, but the basic conclusion is the same -- it's not an error. However, it is neither common nor rare.

During the early 1910s (don't remember the exact dates), the US post office experimented with varied vertical spacings along the bottom few rows, in an effort to reduce wastage due to perforation inaccuracies as the press sheet was fed through the perforator. They experimented with a slightly larger spacing in the bottom couple of rows. Wastage was reduced, but the variable perforation spacing created problems for vending machine companies, so they eventually settled for a larger overall consistent vertical spacing for each stamp. For selvedge collectors, these experimental plates are known as the star plates (variable) and the A plates (larger). When final decision was made, the designation was no longer added to the plate number since no longer experimental.

One of the changes also involved a wider margin along the perimeter. This likely created the stamp shown by the OP. Basically, it is a stamp from the bottom row for which someone (hopefully a selvedge collector) has removed the bottom selvedge. Although not quite the same as "wing margin" GB stamp (which appear on the right/left and are the inside margins of the 2-pane layout), the original discoverer (sorry, forgot his name) of this phenomenon on US stamps also called them "wing margin" stamps. This is why the OP's stamp is so much taller than normal (otherwise, at the bottom you would see the top of the next stamp). My point is, in this case it is deliberate, not a misperforation.