Patent applied for means just that....A=alto.....278474 is the serial number.....L=low pitch (A=440).....viii refers to a neck type used on this model and some 6M's
Pete Hales put together an excellent website with pictures and information about vintage saxes of all makes and models. This link takes you to the Conn Serial Number information page. Click the information box in the upper right hand corner for more great links to Conn information. Pete is extremely knowledgeable about vintage saxes and is a very decent and honorable person as well.
www.saxpics.com/conn/docs/numbers.htm
John