The AAF plate is on the right match rib facing the inside of the cockpit. The Stinson data plate is on the boot cowl, inside the fuselage, about where your left knee goes. It is a bigger version of the component parts, and will have 76-33000 stamped on it plus a four digit number which is the aircraft's serial number.
The C/N on the MLG boss does have a rough correspondence with serial numbers except with the G models. But it is not exact and varies through production. Jim Gray of the Sentinel Owner's and Pilot's Association (SOPA) has a database of this information.
If you are working on the L-5, you need to join the SOPA, at
www.sentinelclub.org. It's cheap and will give you access to a wealth of L-5 information not easily found without heavy research.