@DoubleEggbert56 pretty much nails it.
The other thing is that the Charlton code BC-23b stands for stands for Bank of Canada & the “b” indicates the 2nd signature combo for that series (Gordon Towers 1937 $5.00). For BC-39a the “a” indicates the 1st signature combo for that series (or “Coyne Towers” 1954 $5.00). There were 2 printers for the 1954 series which can either be Canadian Banknote Co (CBN) or the British American Banknote Co (BABN). The printer names are seen imprinted in the lower frame of the 1954 series (& below the designs for the 1937 series).
Pick (or the SCWPM) gave two separate codes for the $5.00 notes P-77 (CBN) & P-78 for the BABN notes. Charlton designated the 2 printers with an “i” so that BABN notes has no “i” at the end of the code while the CBN printed versions have the “i” at the end of the code (so your note is likely a BABN/P-78 note).