I believe this is #313, 18 Plain Junior Blocks in Wagon. See Holgate Toys (1936). Confirmed, found in 1937 catalog. NOTE: Also seen in "The Amarillo Globe-Times", 28 November, 1938.
Plain Junior blocks with the following shapes:
1 Column
2 Doublongs
3 Squares
4 Oblongs
Purchased half-filled with Set of 117 Halsam wooden "American Bricks" and a few other blocks, in addition to the 10 Plain Junior Blocks.
Dating: A similar wagon with multi-colored stars along with the sailing ships was used until 1938. The ship theme was replaced by 1940. I estimate this to be 1936-7. See Holgate Toys (1936) for this same design on a different size wagon.
In 1938, Holgate #1, Elementary Set of Junior Blocks (in box), consisted of:
6 Oblongs (6 units)
4 Squares (2 units)
2 Panels (? units)
2 Columns (1 unit)
4 Diagonals (2 units)
That doesn't seem to work out to 15 units? In at least some other sets, a panel is 2/3 of a unit. Even at a full unit, it's only 13 units of blocks.