Scott R15e, 2-cent USIR orange on green paper, is a VERY tough stamp. It only catalogs about double what an R6e (2-cent Bank Check on green paper) catalogs, but is much more scarce. I own two examples of R6e on document and have seen 3 others that I could have purchased. They will occasionally show up on eBay once every year or two.
Neither Eric Jackson nor Richard Friedberg have had an R15e in stock for years. Eric says that he had one in 2008, and prior to that none since the 1990s. Richard says he has only handled one example in almost 40 years. He puts it at 5-10 times scarcer than the R6e.
To give you an idea of the stamp's scarcity, the high-profile collections of Cunliffe, Tolman, and Joyce did not have any examples of R15e, despite the auctions having multiple examples of R6e (at least there were no R15e lotted as singles; I have no idea if there were any in the remainder lots).
As an experiment, I decided to compile a quasi-census of R15e. If you are aware of any examples that I have not listed below, please contact me with the information. My thanks to Richard Friedberg for his assistance in compiling this list.
I do not have access to a complete run of Aldrich auction catalogs, so there might be others that he offered, not to mention others offered by auction houses who don't have archives online or had high-profile revenue auctions. I contacted Michael Aldrich for his assistance in filling gaps but received no response.
Here are the different examples of R15e I have been able to locate information about going back to the early 1960s, with the most recently discovered at the top. Note: I make no claims as to which of the items below are genuine R15e. Some of them are most definitely bogus, so the actual number of genuine R15e is smaller than the number shown below.
Census # 26. Auction date: April 24, 2022. Schuyler Rumsey Auction 101: The WESTPEX Sale. 2022 Philatelic Foundation certificate. Sold for $X,XXX.XX.
Census # 25. Listing date: March 30, 2022. 2021 APEX Certificate #237916.
Census # 24. Auction date: July 13-16, 2021. Kelleher auction 758. No certificate. Sold for $1,600.00.
Census # 23. Auction date: June 15-17, 2021. Kelleher auction 757: The Crimson Collection. No certificate, but offered with letter of authenticity from Eric Jackson. Sold for $1,400.00. Had previously been offered in Rumsey Sale #92, June 8-11, 2020 where it garnered $900 + BP.
Census # 22. Auction date: August 17, 2017. Kelleher auction 702. 2006 APS and 2017 Crowe certificates. Sold for $1,800.00.
Census # 21. Listing date: February 2017. Found by me in a bulk lot, subsequently received a clean ceritifcate from the Philatelic Foundation, #544484.
Census # 20. Listing date: May 2014. Found by me in an eBay lot, subsequently received a clean certificate from the Philatelic Foundation, #521143.
Census # 19. Listing date: March 16, 2013. Currently in the collection of Bob Hohertz.
Census # 18. Auction date: October 24, 2012. Schuyler-Rumsey Sale 48: The Bulkley Collection. Philatelic Foundation certificate #508668. Sold for $1,300.00 + BP.
Census # 17. Auction date: July 21, 2010. Spink USA. Collector's Series. No cert. Did not sell or was withdrawn.
Census # 16. Auction date: October 19, 2002. Shreves Philatelic Galleries Sale #53: Collector's Series. Sold for $400 + BP. 1984 PF cert. Also offered in Aldrich sale 25 in 1987.
Census # 15. Auction Date: October 25, 2001. Shreves Philatelic Galleries Sale #45: The William S. Floyd Collection. No certificate. Sold for $600.00 + BP.
Census # 14. Auction Date: April 11, 2001. Siegel Auctions. Sale 836. No cert.
Census # 13. Auction Date: December 7, 1991 Michael Aldrich Auctions. Sale 34. 1991 PSE cert.
Census # 12. Auction Date: May 25, 1986. Siegel Auctions. The Isleham Collection. No cert.
Census # 10 and # 11. Auction Date: October 12, 1983. Siegel Auctions. Sale 624. Two examples with one not pictured. No certs.
Census # 9. Auction Date: October 18, 1979. H.R. Harmer. The Eastern Collection. No picture available.
Census # 7 and # 8. Auction Date: October 18-19, 1979. John W. Kaufman. Auction #58. Two examples, neither with cert.
Census # 6. Auction Date: March 3, 1970. Siegel Auctions. Sale 368. No cert.
Census # 4 and # 5. Auction Date: February 13, 1962. Siegel Auctions. The Philip Little Collection of U.S. Revenue Stamps. Two examples, neither with cert. Second example subsequently received a negative opinion from the Philatelic Foundation.
Census # 3. Unknown date. Philatelic Foundation certificate #379103.
Census # 2. Unknown Date. Philatelic Foundation certificate #313206. Subsequently overturned as "artificially tinted", certificate #507351.
Census # 1. Unknown date. Philatelic Foundation certificate #139095.