I have put up this page due to the many questions I get about David Bradley model numbers, and where to find them.
The top number is the model number of your tractor. This number can be used to reference information about your tractor on this (and other) DB sites. The pictured tag is for a model 917.575138 tractor. The first six digits (917.575) are the same on all David Bradley tractors. 917 is the Sears source code for David Bradley corporation (at least it was at that time), and 575 seems to indicate "walk-behind tractor" or something similar. The digits that follow these six (138 in this case) are unique to each particular model.
The lower stamped number is the series number. It is NOT a serial number. Series numbers were assigned to batches of tractors. Hundreds (or thousands) of machines may have the same series number. Because of this, the series numbers are of little use for dating purposes, although they can to some extent place a tractor in the early, middle, or late part of the run for a particular model of tractor.