Hey,

I have owned two 586's in the past but I don't recall seeing any thing like these stampings before.

First, we are talking about a 586 - no dash. There is no "M" stamp.

The serial number is in the customary place as well as the "Mod 586" right beneath the serial number. The serial number is AALxxxx.

Here is a description of the stampings I have not seen before - On the other side of the cylinder pivot, just opposite the cylinder, is stamped;

C11
A23
28638

That is exactly how these stamps appear.

What the heck am I looking at?

Thank you.

They're known as "assembly numbers" and are used during the manufacturing process to keep track of fitted parts that may be separated for certain operations. They have no significance once the gun leaves the factory.

Am I getting forgetful and have seen them before or is it a case of "sometimes you see them and sometimes you don't?"

Maybe more like "sometimes you notice them and sometimes you don't." I can't say they appear on all Smiths, but they're certainly there on most (perhaps all - not sure) of the ones I own.