First off I have never been very lucky at making "finds", so I am posting this with a bit of skepticism.
I was sorting through a batch of 2 x 4 "cuts" just seeing if there was anything interesting enough to keep intact and not soak. I had a pile of 1c greens and 2c reds from the mid 1920's-late 1930's. My usual method of going through these is to first sort by set, then face value. Then I start in with the perf gauge. I normally only check the vertical edge (maybe this is bad?) to speed things up. The ones that measure perf 10 vertically also seem to measure perf. 10 horizontally... so perf. 10 x 10. The ones that measure perf. 10.5 vertically, all seem to measure perf. 11 horizontally....so perf. 10.5 x 11. The ones that measure perf. 11 vert "usually" measure perf. 11 horizontally as well...so perf. 11 x 11.
....until I found these.

They were in my perf 11 x 11 pile, so naturally I check them to see if any are also rotary press coil waste issues...yeah right!
But these actually measured the 19 3/4 x 22 1/4 (approx). I thought to myself, "nah it can't be.' So I got a known flat plate copy to compare and they are definitely taller. I also compared to a common rotary press copy and they were very close. Could I possibly have a vertical pair, on piece, of the very rare rotary press coil waste (sc 594)?
Then I decided to recheck the vertical perf's and the recheck the mm measurements. They were still the same. But, then I decided to check the horizontal perfs and they're not 11 at all... they're 10!
So these are, in my opinion, perf 11 x 10, 578's.
Am I missing something else. Not the ultra rare variety, but if id'd correctly, one of my better discoveries from a batch of common items.
Thoughts?
Unfortunately, the top stamp in the pair is torn across the bottom and the bottom stamp is a bit heavily cancelled. But still exciting enough to have me up this late making this post. Again, assuming I've id'd them correctly.
