I had also posted this some time ago, but decided to update with some new photos. This is the #33 Mechanical Horse and Five Assorted Trailers, and was made only from 1935-1937. After that Meccano decided to drop the cargo trailer, so it was still numbered 33, but was now the Mechanical Horse and Four Assorted Trailers. I bought this many years ago from Keith Harvey, and he said that this was the nicest version he had of this classic little set, so it is the prize of my Dinky collection.
What is really wonderful is that there is absolutely no sign of fatigue in any of the pieces and even the original white tires are in great shape and not even cracked. Later versions had the petrol tank trailer with the Esso name on it. The box is also in excellent condition, so apparently this little set was packed away for many years and never played with, so no wear at all.