Impressive.  Did you see the PA $5 plain back charter 6301 SN A109273 pp A4 error note with SN A109274 on back? Just awesome! thanks!!

Here it is! Quite an amazing collection. I imagine if this collection went to auction it would be in the 30 - 50 million range. 

That's the one!  It surprises me that the sheet wasn't significantly damaged including this note from the A4 position (this note).  At this point in time sometime after 1925 since there's no treasury SN, the Mellon NB had its officers' signatures engraved on the front plates, so I have to wonder if the sheet was ever sent to Pittsburgh.  I suspect it was not and somehow escaped destruction.  Possibly it ended up in the Treasury Secretary's hands as that would have been Andrew W. Mellon.  I wonder where notes with plates B4 through D4 ended up.  Possibly they were destroyed having been mangled by the printing machine (which accepted sheets of 4 notes at a time).

I too am astounded. How in the world would the Ch#, seal and serial number be printed on the reverse of the note in such and awkward position? Mangled is the operative word here as the printing machine must have jammed to create such an error. I wonder if the sheets were re-fed into the machine or if this error was a single pass. The Bebee collection is fascinating. The National below is a: 1) territorial note 2) high denomination 3) error, a $50 reverse with a $100 obverse in addition to the CH# being inverted in addition to high quality!

If you search Heritage's archives (search $50 Brown Back in error notes) you find this info in the description of an Eric P. Newman $50/$100 BB from Kansas City. In June of 1945, Barney Bluestone sold the Grinnell $50/$100 and the $100/$50 Brown Backs from Albuquerque, New Mexico Territory in one lot. The price realized was $830, at the time a very high price. The pair later found their way into the collection of Amon Carter, Jr. and, upon Amon's passing in 1982, the pair was sold privately and has not been seen since--until now that is.

When the charter was printed upside down on the back sheet, it wasn't recognized, but was used to orient the back for the printing of the fronts (can forgive the mistake given the charter is in blue and much larger than other design elements on the back).  This is what you will also find on the Kansas City $50/$100 BB.  

Heritage also sold the Albuquerque $100 BB two sheets prior which is not an error note--they sold it twice.   I also noted the Cashier was the Assistant Cashier (R.A. Frost) on both the C on L error above and the B592834 note.

Reminds me of the inverted Jenny airmail stamp, only it didn't receive the hype.  Too bad I wasn't in Albuquerque in 1903 in need of some pocket money :-)