Error in bus-stops-dat, for the Post Box, intro_year and month given twice. The second one (1842) is closer to the actual introduction dates (1852 in Jersey, 1853 in Carlisle).
Obj=building
Name=LoadingBay
# must be 1*1
type=stop
waytype=road
intro_year=1750
intro_month=1
intro_year=1842
intro_month=7
p.s., if I get ambitious I might even update the icons and do a green postbox which was the standard from 1852 to about 1874.
How about using the Coaching Inn graphic for an add-on building in the early years? That would work for ports as well as roads.
Better late than never. Here's updated bus-stops.dat and bus-stopwithmail.dat to have pillar boxes only become available 1852 in green version, and the red version in 1874 (see: http://en.wikipedia.org/wiki/Pillar_box#Moving_towards_a_standard_design (http://en.wikipedia.org/wiki/Pillar_box#Moving_towards_a_standard_design) ).
This does leave us with only the Post Office from 1844 (when the Staging Inn retires) until 1852 when the pillar box becomes available.
Likewise, there is a period of road frustration between when Mail Coach retires in 1840/03, leaving only the rudimentary mail cart (6 km/h, capacity 4) for 43 years until 1883/11 when the Parcel Coach (18 km/h, capacity 25) is introduced.
That's rather splendid!