I'd be interested to see a bona fide reference to your statement that all USA auto Amazons came with the 4.56 axle. This contradicts all of the info I have, including the Volvo green books, brochures and other reference books. Certainly the Wagon with auto box had 4.56, but that was the standard ratio for all B18 wagons. The wagon only had the auto for 1967, the last year of the B18 before the change to B20 in August 1968.
The BW35 is listed as being fitted from the 1963 model saloons and these all had the 4.10 axle. This option was probably introduced for the USA, as very few cars with auto boxes were sold in Europe at the time.