Let's start with the 866, that appears to be a date; Aug. 1966 when it went back to S&W for a rework.

At that time the sideplate was either replaced and if so, Smith would have re-stamped it with a matching assembly #22600. More likely however, the original sideplate was stamped with the current 1966 large logo on it as part of the refinishing.

The diamond on the grip frame indicates a rework.

The 61 and L2 are normal inspection stamps, likely original.

The protocol in 1966 was to blue the ejector knob.

You need a historical letter, although it won't speak to the rework, it's necessary before you can request rework documents from the S&W Historical Foundation.

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Jim
S&WCA #819