The #1 529 4 with an N above the 4 is an assembly # and the N an inspector stamp.

ASSEMBLY (factory work) #s: These multi-digit numbers of 3 to 5 digits, are on the yoke at the hinge, in the ‘yoke cut’ on frame (accompanied with a stamped inspector letter) opposite the yoke near the hinge, and inside of the sideplate, for the pre war and early post war period.

Once the gun is shipped, the only use for the assembly is to confirm the three parts it's stamped on are original.

In 1957 the assembly # in the yoke cut of the frame was relocated to the left side of grip frame after model #s were assigned and the serial # was eventually added in the ‘yoke cut’ where the assembly #, now moved to the left side of the grip frame, used to be. You know they are assembly (factory work) #s because of those 3 locations that always match on guns that are original, and that’s the only usefulness for them after guns leave the factory; still used to this day, long after serial number locations decreased.

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Jim
S&WCA #819