Sorry Dan for stealing your title.
Anyway, I recently picked up a lot of RN checks pretty much sight unseen (one of those auction lots where you have to rely solely on the description.
Here are some of the better ones that I have found.
RN-D1 Rocky Mountain National Bank Bill of Exchange

RN-B1 First National Bank of Helena

RN-B1 Kountze Brothers & Co. Bankers (Unlisted by Castenholz)

RN-D1 Deseret National Bank with Vignette of Eagle and Beehive (Unlisted)

RN-B1 Colorado National Bank with 1866 Vignette of Denver
Dated September 14, 1868 before Colorado statehood

RN-G1 Colorado National Bank with a Different Vignette of (I assume) Denver
Printed by National Bank Note Company (Unlisted)
Dated April 24, 1882 after Colorado statehood

RN-D1 First National Bank of Georgetown Colorado Printed by Western Bank Note & Engraving Co., Chicago (Unlisted)

RN-X7 Vindicator Consolidated Gold Mining Co.

RN-C13 Printed in Salmon? T. J. Carter & Co. Bankers, Golden City Colorado (Unlisted)

RN-C1 Also printed in Salmon? Merchants & Farmers National Bank of Quincy Illinois (Unlisted)

RN-X7 First National Bank of Manchester (NH) (Unlisted)

RN-K6 First National Bank of Denver
Interesting back story on D. H. Moffat, Jr.
https://historicdenver.org/wp-conte...-history.pdf
RN-B1 L. H. Hershfield & Co.

RN-B1 L. H. Hershfield & Bro.

RN-B6 L. H. Hershfield with and Co. Handwritten in Ink

RN-P5 Rocky Mountain National Bank Certificate of Deposit

RN-Q1 Rocky Mountain National Bank Certificate of Deposit with Stamp Redeemed Reading Up as the tax had expired by September 23, 1873 and the tax was refunded

RN-B16 Oregon and California Rail Road Company Check signed by President Ben Holladay

RN-B1 on a very attractive First National Bank of New York Jay Cooke & Co. Bankers Dated August 8, 1873 for $150,000 ($3.9 million in today's dollars)

RN-B1 Cayuga County National Bank of Auburn NY with Metropolitan Crossed Out and The First printed in Red

I'm certainly no expert on these so comments are more than welcome.