I added another 930 to my collection recently, a new-old-stock 930 JM Pro (24", p/n 85118) - it's dated 2021-02 on the box with a serial of AF0077xxx
Look's like your 500a is one of the newest we have in our data base so far.Mossberg 500 (12ga): V1460xxx. I picked it up a few weeks ago, used, with no box.
Shockwave (12ga): V1321xxx. Bought new about 3 years ago.
That makes me feel good about the 500. I picked it up off GunBroker for $280/shipped, so I feel like I got a pretty good deal.Look's like your 500a is one of the newest we have in our data base so far.
The shockwave is right behind it. Educated guess it's around a 2021.
Thank you for posting your numbers. I'll update our data base.