The 1861 - 67 12¢ Washingtons

I am curious about this stamp. It has a very wide margin on the right but no perforations. Is this just a scissor cut? I can't find any reference of a imperforate 12c Washington. Thanks!

Due to the size of the right and bottom margins, experience has shown me that this is the bottom right stamp from the left pane. Right and left panes are cut apart and have straight-edges. The bottom, however, would have a piece of selvage on it, perforated between the selvage and the stamp. In order to be considered an 'imperf between', it must be a pair of stamps with no trace of perforations in between them - those are rare. What you have is not rare. It might be considered 'scarce', in the ' ebay' sense - over described. Considering the size of the two margins (and the third one on the left) your stamp is certainly interesting. The margins are certainly larger than typical, but not unique. Nice jumbo! The holy grail would be to have the stamp centered a bit lower, enlarging the top margin, so all 4 margins would be jumbo margins.

FYI: Notice the tiny dots off the lower right corner. These occur on the bottom row and are used to layout the sheet when the plate was made. More evidence that this is from the bottom of the pane. You can even see a similar dot off the lower left corner - this one would have been used to help locate the column of stamps to the left of this one.

It is a straddle pane margin. The right side was the edge of a pane.

Thanks for the information. Greatly appreciated.

it always amazes me the knowledge members of this forum possess.

Sellers on ebay sometimes inflate definitions of terms to enhance the desirability of what they are selling. Calling this stamp a "straddle pane single" would be an example of that.
Straddles do indeed only occur along the center gutter between adjoining panes on a 200 subject plate, and they do serve to point out that such a stamp came from one of the positions along either side of that gutter. However, the term "straddle" is usually only applied to a stamp which shows a bit of printing along both sides of the gutter, thereby effectively "straddling" the gutter.
A straight edge along either side with a wide unprinted space between the edge and the printing does not necessarily come from the center gutter. All of the stamps along the outside edges of the plate were given extra outside margin. When the perfs are set wide enough it is sometimes possible to trim the outside perfs and still leave the kind of wide margin most often associated with the center gutter.

Let me suggest that unless you see printing on both sides of a wide straight edge, do not call it a "straddle margin stamp" or pay a premium to anyone else who does that.