Some pictures
rear non-slot tank
http://www.a7a10.net/BSA/bsapics/BSA11.08.04.jpg
rear slot tank
http://www.a7a10.net/BSA/bsapics/i-6_B_L.jpg
+ on page 13 in "BSA Twin Restoration", there is a picture of a 1951 w smaller chrome tank with apparently no slot for the seat, looks like an earlier A7 tank size.
Then, in Bacon's "twins and triples" I find the pic of a non-slot tank on a 50' plunger, so now I am confused! Anybody know the exact story and dates?
Most likely one of the 1000 errors in every Bacon book.
You should treat them as coffee table books not reference material.
The only worthwhile parts of them are the apendicies at the back.
All he did was add a bit of text to link BSA factory press releases together.
Most of the photos are from the same source so they are what the advertising department said the factory was going to make FOR THE UK MARKET next season and not necessarily what the factory actually made.
OTOH the tables in the back of the Restoration Guide which were compiled from the parts books would be a lot more use to you.
Again it is UK SPECIFIC but it does clearly show when a new tank was introduced and how long it got used.
You also have to remember that BSA's production time preceeded computer controlled inventory and BSA regularly ran out of parts
So the fitting of substitute parts was not uncommon, be they parts from a different model that would or could be made to fit or parts bought in from an external supplier.
BSA made their own tanks, but if the presses were set up to make Bantam tanks when they ran out of A 10 tanks, the last 10 tanks needed to fill the order currently being made would be sourced externally , if they could not be pinched from the spares warehouse.
Considering just how easy it is to damage a tank and A 10's are now old enough to draw a pension there is no reason to believe that any bike is still wearing the same tank it left the factory with.