Owen_Dyneto wrote:
Engine side of the cowl. 22nd Series and later it was placed over the heater box location; in earlier years it was generally at the top center of the cowl. Six digits beginning in 1929 with 000001; starting with the 24th Series they were 5 digits preceded by first an "A", and then "B" and "C", ending with "D" in 1956. Not used in 1957 and 1958.

Here's some detail. Note the unusual representation of the digit "1". The factory print misrepresents the shape of the character "D"; in practice it was much more square and as the dies became worn and indistinct by 1956 it's sometimes difficult to tell a "D" from an "0" which also has a squarish shape. The triangular end brackets were not used on the junior models 1935-1937 and thus for those 3 years there may be some duplicity of numbers, though distinguished, senior or junior, by the presence or absence of the brackets.