Help Id. 4cent M. Washington 3 Imperf. Sides

So I found this stamp in with a bunch of other stamps, I have looked in the Scott book and on the 1847 site. Can any one help me, why are 3 sides straight? Is this a private perf? What would the Scott number for this be and what is cat. value? Thanks!

It is a coil stamp with the perforations on the left trimmed off

To me, it looks as if it were applied by an affixing machine that cut the perfs off on the left side and left a generous margin on the right.

There is a brown line that runs through the perfs. on the right side. I don't see this line on my other 4 cents MW stamps.

This was originally the left stamp of a line pair. This is a rotary press stamp; rotary presses were made by taking two flat plates, curving them and then welding them together. The line is from the space above the weld, so a line pair is always two stamps from different plates. That is why 599/599A pairs exist.

If I find a line single I usually keep it.

As I recall, the rotary plates are not welded together when they are affixed to the press. They are merely mounted in with special clips behind them holding them down, and they butt up against each other with a straight-line gap between them. This fills up with ink and this "joint line" is printed onto the stamps. Hence, joint line pairs. I saw a setup for this operation as a demo several years ago, and that press was fitted with two plates, which looked huge to me.

What great information on this! Thanks so much!

The name you're looking for is grippers. They engaged machined out portions of the plate. Stress could result in "Gripper Cracks - cracks that formed over slots cut in the edges of the engraved plates curved to fit the rotary press cylinders at the spot where the clamps held the plates to the cylinders." These cracks are a variety of plate cracks that can appear on stamps and are desirable.

Source for attribution: http://www.1847usa.com/Glossary/G.htm