What The Public Got
Did BSA ever do more than keep the twin carb option open? I believe they did, but if you think I've been building a case based mostly on emails from people I'd never met in the US and Canada, then consider the following.
Authorised Accessories in the USA
I have a copy of an American dealers sales brochure showing a range of performance parts for pre- unit BSA twins, approved by BSA and bearing BSA part numbers. Missing images (×)
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The A10 parts are the same as the ones described by people in the US and Canada so they were probably supplied as after market tune-up accessories, in the same way as Dresda and Rickman in the UK produced parts to make Triumphs and Nortons go faster.
Brochures produced in different years list
parts and prices as follows;-
A7 twin carb head
A10 twin carb head
Manifolds for above
Two carb kit including the head, manifold, twin throttle & cables, nuts, washers, studs and two monoblocs, one with cut-off float chamber
This raises a couple of questions. First, did the approved status mean that BSA would create a part number for any new equipment manufactured by/for the dealer who was not original issue for that model of bike? Second, the 1954-56 A7 alloy head is listed here as a twin-carb part. Since it at least is listed in the 'official' BSA parts lists as a single-carb head, why would BSA approve what appears to be fraudulent advertising?
To find out, read What were BSA Up To?