Pretty close. The Letter prefixes are just part of the normal sequential progression in order to keep the serial numbers 5 digits long. 10000-19999, A10000-A19999, B10000-B19999...etc. Followed by Z10000-Z19999, AA10000-AA19999, AB10000...and so on. Maltby got up to BZ prefixed numbers by 1945, I believe.