Ok, now we have mostly enough to identify this.
It's a flat plate printing, we can see by the bits of green ink on the back of the stamp.
It's either Perf 10 or Perf 11, I'm going to guess 11, just based on view.
That matches only 1 stap, a Scott #498. It has a catalog value used of about 35 cents. It is one of the most common varieties of the 1c Washington. It will also be without watermark.
Of the slight chance I'm wrong about the perf, the other possibility is perf 10.
That would make it either a 424 (single line watermarked) with a CV of 25 cents or a 462 (no watermark) with a CV of 35 cents.
If you get a perf gauge and it perfs 11 you know what you have.
If it perfs 10 then you need to put it in watermark fluid to make a positive ID.
The pefs at the left (front) of the stamp have simply been cut with a scissor, probably when someone opened the letter it was originally on...
Edit: Picture added below to show your perfs are the same at top/bottom as left/right so it is NOT a perf 11 x 10...
I used Photoshop to copy the perf from top and aligned it to the side, they are identical.