Set 5, 1929.
The basic box layout exists in oak finish to start with, and
this must have just been for one year, 1928, when the boiler etc found its way
into the set. It looks like it changed very soon to blue enameled as it is
listed as such in the 1929 production catalogue. At this point it had a blue
case with blue tray.
The tray was changed to yellow in 1933 with the advent of the red/green boiler etc. The same blue/yellow box became the H set option from 1934 on - probably until the end of the letter sets.
If the contents are original, the ship's funnel is the key item. According to contents lists, it only appeared in the 5 in 1928 and 1929. Since 1928 should have had an oak box, it is estimated that this is a 1929 set here.
Photographs courtesy of James Bobyn.