The track locks were added to the 1949 set, it is a content item not in the 1948 set. My boxed 5114 set has a 312 engine and green passenger cars despite the catalog listing the set with red cars. Green cars in a 4909A set would be unusual.
I neglected to ask about the set box. Many have date stamps applied by the manufacturer of the box. Also in the 1948 to 1950 time frame there were inspection slips in the box. I have these in most of my early sets and they also allow determining the approximate day the set was packed in the factory.
The 314AW was last made in 1950, the 315 appeared in 1952 only.
