The start of the war in 1939 saw the British Army faced with a shortage of basic kit -- where have we heard this before?
'39 pattern webbing was adopted as an interim measure, Since there were problems with production of '37 pattern kit. In essence, '39 pattern is to '37 pattern what '14 pattern is to '08 pattern.
This particular pattern of webbing is very little seen - basically, it is like '37 pattern in leather, except that the ammunition pouches are equivalent to the '37 pattern engineer's pouches, which hold 2x2 five round chargers on each side - since the large ammunition pouches would have been an absolute disaster.
Apparently, the ammunition pouches were highly sought-after later in the war by snipers, who would use them with the hanger cut off to hold their ammunition.