Some of the sewing machine perfs are tough as well, e.g., the R120. I think Richard said he's only aware of two examples extant. The R115a is actually about an order of magnitude more scarce than the R112b, despite cataloging $50 less. I've seen a grand total of two in person, one of which I own, and the other I traded to Richard. There were a couple of multiples in the Cunliffe collection, but outside of that, nada. Siegel has had exactly one example in their history.
I have several examples of the R19 sewing machine perf, but am still looking for an RB3. There was one that actually showed up on
ebay last year, but I didn't see the auction until after it was over. The stamp went for full Scott, which isn't surprising. That's another rare bird.