404-5 carb jetting differences

[GlennBurnage]

According to the 404 - 405 manual the centre carb is set up differently from the front and rear on 404s and early 405s. The differences are in the main jet and the air bleed. There is also a difference in the carb designation numbers -
1705 (centre) and 1704 (f & r) I wonder if this is for the actuating mechanism attachments? But this is not the case on the later 405s which have the same designation for all 3 carbs -1763, and the same set-up, jet etc wise.
The only other Bristol engine details that I have are in the Bristol 100C, D & D2 series manual downloaded from the ACOC, this states all carbs to be the same designation and all jets etc the same.
Can anyone shed any light on why there are these differences and why things changed during 405 production?