This is a 1928 US Number 3x outfit. The brown wood box was last used in 1927; the wood box was painted green starting in 1928. But what, you ask, is the "x" after the number? Starting in 1915, when the low voltage electric motor was introduced (see the page on this site on motors), an x was added to the No. 1, 2 and 3 outfits if a motor was added. The larger outfits were sold only with motors, and it was deemed unnecessary to add the x. In 1928, though, an x was added to all outfits with motors. Beware! There was an earlier 5x outfit: it was a Number 5 outfit with the contents of an Inventor's Outfit added.
It appears that 1928 was the only year that the Sector Plate (part No. 54) with three rows of holes was used. This version was introduced in England in 1927, but apparently did not appear in the US until 1928. In 1929, US outfits seem to have reverted to the single row of holes.