What Kind Of Stamp Is This?

This is probably a real rookie question but I'm unable to figure out what kind of a stamp this is (with one vertical side looking knife cut) - any suggestions?

Looks like a regular stamp. Could you post a closer scan of just the stamp ?
To answer a bit better, these stamps were printed in large sheets and then cut into four smaller panes, hence the straight edge.


Peter

Thanks Peter, here is a close up of the stamp:

The U.S. Post Office Department at the time called stamps of this type the "ordinaries."

The stamp in question is a US Scott #213 (first issued in 1887). Yours was used in 1888. These were printed in sheets of 200, consisting of two panes of 100 side by side and joined in the gutter between them which was not perforated. This was cut vertically into the two panes. Same idea as Peter suggested, but the number of panes in 1888 is half of what he had suggested.

Thank you both! It's the little details like "not perforated" at the gutter which give context to stamps like this