the friction surface of all drums will be wider than the shoe width. note the large rust ridge present when you take drum off to replace parts ? when drums are turned, that ridge becomes part of the "friction surface". a 2.93 friction surface is for 2 1/2" wide brake shoes. 3" wide shoes would lock that drum up solid when the wheel was bolted on no matter how the shoes were adjusted.

edit : brake shoes, as a rule, use the same shoe core for both front and rear applications. that is why there are unused holes and shapes present. "most" shoe material is also the same [unless you are talking a performance shoe from a company that deals with specialty lining], and both sets have a primary[short] and secondary[long] lining, where the primary goes to the front, and the secondary goes to the rear.