The Base 'A' in your picture is what I consider as the very first earliest full production type, I have not found one earlier than this and it is the same as the one shown in the picture by "Fox".
The Base 'A' for most collectors and shown in the White Book as... 'BASEPLATE TYPE A' does not have the 'CAST STEP' directly in front of the engine sump, the area between the sump and front axle.
Here is a picture showing both types...

- Base 'A' & Base 'Aa'..JPG (100.25 KiB) Viewed 961 times
I am not sure how these bases will be re-numbered, but as you can see the base on the right is 'Aa' for no other reason that it fits between 'A' and 'B'. I expect if the bases stay as 'A', 'B', 'C', 'D' etc, then they just need to be moved one place along to include the 6th type which will be the new 'B' type, the 'A' type with the step.
GHOSTHUNTER.