As far as I know all model 56 had reverse and I have not heard the theory that is what the A indicated. Serial number 560101 was delivered on 24-1-1949 to P Harsham, Coddington, Ledbury, Herefordshire.
The 56 is 8hp, 2 speeds which are selected via pins in road wheel hub, the long pin is inserted on left for low and right for high gear.
I have operators manual and spares list which I can e mail if you send me a pm with your e mail address. I have never seen a workshop manual for these machines. Original engineering drawings are at MERL, Reading, drawing number is usually same as part number.
Petrol/oil mix is 6% and gearbox oil should be straight 140, I use Morris Oils AG140.