How can I tell if my L.C. Smith shotgun has a regular frame or a featherweight frame?

It is relatively easy to determine frame size by looking at the lock plates.

1. The upper corner of a lock plate from a regular weight frame is somewhat rounded, while the upper corner of the featherweight frame comes to a point.
2. While assembled, the left lock plate of the regular frame shotgun has one screw head and the left lock plate of the featherweight frame has two.
3. When removed from the gun, the right lock of the regular weight frame has four holes, while the right lock plate from the featherweight frame has five.
   
   

Lock plate from a regular weight frame below and a lock plate from a featherweight frame above.

(click image for larger view)

More detailed measurements may be found by viewing the engineering drawings of the regular frames and the featherweight frames.