According to Bacon, no 18/42 combination offered stock in any model or year and no 20 or 21 tooth gears available back then. I guess those came later as aftermarket pieces. It seems to me that your bike must have been 19/42 originally and someone changed to the 18, or, if your sidecar rear was 49, Bacon shows "A7ST s/c" with 18/49 for 1949-1954, which might explain something if your bike started life as an A7. BSA might have just dragged the gear ratios along without making it a matter of record that they also applied to A10's, as well.
For the most part, in all of Bacon's books, I think he just recites from the records without adding special information. Kind'a like me, right now...and, no, I am not a librarian in real life or on TV, but I am just reading from the books.
Richard L