1954 Fives with letter "C" Proceeding Plate Number

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I was going through some 1954 $5.00 notes, and I had not noticed until now that the front and back plate numbers on most of these have the letter "C" proceeding the number.
This is the only denomination that the plate number also includes a letter.
I also have a note with the prefix P/C that there is no letter in either the FPN or the BPN.
I'm guessing that all the notes printed by BABN have no letter proceeding the plate numbers and all the CBN notes have the "C" added to the plate numbers.
It appears that the 1954 five is the only denomination and series that a letter was added.
Was there ever a reason given for adding this letter to the plate numbers on the 1954 fives?

Cheers, Bill

I can't find the reference on this subject - probably in the CPMS Newsletter or Journal - but you're essentially correct.  1954 $5 notes printed by BABN have no letter, and those printed by CBN are prefixed with a C.

I'm not really sure WHY this was done though as the plates already have BABN/CBN imprints at bottom centre, and nothing similar was done on the other denominations, particularly the $1 which was printed by both companies simultaneously.  The $5 notes were a hard cutover early in the R/C prefix.