If it's indeed the set that the schematic shows, it's 1937, and the knobs are the same as others built that year. Sears owned Colonial and had a lot of them built by Colonial and Stewart Warner in those days. The tubes have two-volt filaments per the schematic, and also shows that it takes an external wire antenna. If there is a model number anywhere left, the first three digits before the decimal point indicate the manufacturer. A list of the manufacturer's codes is on this link:
http://antiqueradios.com/forums/view...ht=sears+codes
Reece