A157 2c Washington Perforation Error And ID Help

In sorting through my dad's old accumulated stamp piles, I found this 11 perf Washington perforated on 3 sides, and imperforate on the fourth, but you can see the edge of another stamp printed on the imperforate side. There's no indication of any holes or cuts in the paper using 20x magnification. I read that there's a rule that it's flat plate if the design doesn't measure 19.5mm. I cut corners off a 10.5 perf rotary stamp and it lined up, but it's width looks just under 19.5mm, so... am I correct in identifying this as a 554 with a perforation error? Thanks in advance for any feedback!

Your stamp is from a booklet pane, which were often miscut enough to show the edge of the stamp in the next pane. The straight edge is a natural occurrence. A curiosity of no added value.

Thanks, John!