My fault on the picture. That is the only one I have and my camera is down. Ther is a current auction on Ebay with markings just like the one I am talking about.
I believe the mark shown is a identification number that was added by a unit armorer. I've seen bayonets with various types of markings like paint, magic marker and electic pencil etching. The individual units were responsible for keeping track of their own inventory.
That is what I suspected, except that when we were marking our weapons, we used a dull yellow paint. I have just never seen a unit number or ID number actually stamped into the blade like this. It just seems unusual to me.