Earlier ones sometimes repeat the SN on the bottom of the barrel, newer ones often repeat it (on the larger frames) on the frame near the model number behind the crane which would be visible when the cylinder is opened. The SN on the grip frame is engraved on the bottom, there may be other numbers along the sides which were for use in the factory.
SN on bottom of grip frame.
48-4 SN #1.jpg
SN repeated behind crane
48-4 SN #2.jpg