Hello,
I now have TWO SIG P365 XL pistols. I’m trying to figure out the dates of manufacture. One of them is very slightly fired, no box, no extra mag (the way I bought it today). I bought my other one a couple of years or so ago. My guess is you can tell the manufacture date by serial number? The one without the box is 66FXXXXXX. The new in box one is 66AXXXXXX. What are the dates of manufacture? Thanks.

Man, you are giving hardly any info here. Just 66A and 66F with a bunch of Xs after. And, I don't understand how a "new in box" gun has no label with date info on it. Best to call SIG and ask.

I'm almost certain the one w/o the box has a serial# starting w/ 66B. The P365 series began in Jan 2018 w/ serial numbers starting w/ 66A. The six digit number after the prefix leaves room for 999,999 units. Sig might be up to prefix 66C now.

Call or email Sig w/ your serial numbers, and they'll provide you w/ exact manufacture dates.

The new in the box gun does have a label 27-JUL-2020. That answers that. But the one without the box, the plastic frame has a date dial 12/21, so I guess that is the date or approximate date for that gun.