Weird depressions inside of slide

[PAFrank]

I have a stupid question about the slide of my new M&P. Inside of the slide toward the front, (where the barrel sits), I noticed three small circles. They are depressions, seem to be machined in. I don't know why I did not noticed them before. Are they suppose to be there? If so, does anyone know why they are there? Sorry, I'm not able to post a picture. Any information would be appreciated, thank you.

 

[presspics]

Yes, they are supposed to be there. I believe that the number of depressions signifies either what revision of the slide or who the machine operator was.

 

[GaryM&P]

It identifies the machine that milled the slide.

 

[BobRowe]

I noticed that also in my new M&P 45. My M&P 9 and M&P 9c do not have them.