Apparently you got the model # from the engine. The model # of the tractor will be on a brass plate on the side of the transmission. It will start with 917.
Sears used model #s starting with 500, on Briggs engines that they used on some of their equipment. Sounds like your DB still has the original engine. Tractors with 5 HP, or larger engines, did not have a hood. With the smaller engines, they usually had a hood, but there was an economy model offered, which had no hood.