This Is Cool (To Me) #158

So I'm finally able to start digging through my boxes of letters/postal cards and etc. that I inherited again. Found this today, I know it's a common stamp and what I think is cool is also common, but check out the "cut line mark" (my words, I'm sure there's a technical term for it) on this stamp I'm calling a #158 (no grill observable, can't tell for sure if the dark shading secret mark is there or not, and the letter inside is dated 1877). Pretty cool to me that somebody somewhere missed perforating this stamp on one side. What do you think?
Dave

What you have is the right stamp (10th stamp) of the first row of a left pane of 100. At the upper right is the arrow that was used to align the sheet of 200 for separation into two panes of 100. The right side is a naturally occuring straight edge (imperforate). The left pane will have 10 stamps with a natural straight edge at the right and the right pane will have 10 stamps with the natural straight edge at the left. Position 100 of this left pane probably showed a similar arrow pointing up at the lower right. There were typically two arrows on the unseparated sheet of 200 one pointing down at the top and one pointing up at the bottom.

Wow, that's really interesting. I guess I was pretty close with "cut mark" (as I was referring to the arrow). Now I'll have to keep my eyes out for a straight edge right or left.
Thanks for the information, wt! Much appreciated!

The term "straddle margin" is generally used to describe a straight stamp which includes capture of some of the design from the adjacent stamp in the other pane cut from the sheet.

The Stamp Collectors handbook
Samuel Grossman