The 500 was first sold to the .mil in 1979. The 590 was in 1981. The A1 was after that.
If yours was a police trade in, that is probably all it ever was.
The vast majority of Mossbergs sold to the .mil are and were the 500s. A lesser number of 590s were sold, and even fewer A1s. The A1s were a rather limited purchase item, and don't see widespread use.
In terms of upgrades: yes, there are a few you can do. In terms of the safety, the factory slider is fine. The Vang offers a slightly wider slider that is a little easier to manipulate. Do you have to have it? No, but it is a nice improvement. Another good safety is the ETA safety.
The mag springs in OFM shotguns are notorious. I would recommend you getting a couple of Wolff extra power springs. They are cheap, and it will ensure proper functioning.
The followers are usually pretty good, although you may want to polish or smooth up the outer bearing surface of your follower and the inside of the mag tube. You don't want to do too much, just make it move easily.
You have a bead sight gun. If you want to improve the gun in that regard, the XS Big Dot tritium is a fantastic bead sight replacement. Mossberg beads are a little small, and the Big Dot is oddly enough very big with a large white outline around a tritium vial so that it is very obvious in daylight and dark.
The Speedfeed stock is ok. If it has been on the shotgun since 1998, check the recoil pad to make sure that it doesn't bow out in the middle between the two recoil pad screws. The SF stocks from that era had some problems with that. The newer ones haven't, but I don't remember when the stocks were improved.
You have or should have sling buttons on it already if you want to run a sling.
If this is going to be a defensive gun, you may want to look at means for mounting a light. Those options range from a Surefire weaponlight to a railed forearm with clamping pistol-type light, a barrel clamp light mount, or one of the new CDM light mounts that goes under the mag tube cap. If you are going to run a light, then you should definitely get some training to use it.
You can pay someone to polish up your trigger group, or you can go shoot it. Either way will work. Having it done will just speed it up.
Since your gun was made in 1998, there have been several upgrades to the pins and springs in the trigger group. I have one that was made in 1999 or 2000 and sent it to Mike at Aimpro to have him replace the pins and springs and also smooth it up. My trigger had worn in pretty well, but since I was having new parts put in already I had him smooth it up too because I didn't want to miss having a nicely worn in trigger. I'm actually very glad I had him do this because the trigger feels great, and I don't have to start all over with it again. This is what my trigger group looks like with the new parts and polishing from Aimpro:
The shiny parts are what Mike polished for the most part, as quite a few of my trigger group parts were replaced. However, normal wear will result in a very similar appearance although the feel will be a bit rougher/sloppier. You could do some of the polishing yourself, but I would really recommend you let somebody who is familiar with the platform do it if you are going to go that route. The trigger group can fall apart pretty easily if you pull the trigger and release the hammer. In fact, if the gun has been well-used it is possible to have the pins fall right out of the trigger group if it is held on its side with the hammer forward.
Since your shotgun is a trade in, there are a few things to keep in mind. It may have had quite a bit of use, or almost no use. It is possible to have a lot of surface wear that may lead you to think that the shotgun has been heavily used when in fact it is all from riding around in a vehicle or being banged around on a limited basis. Check it internally.
Also, since it is a trade in I would very carefully and completely clean it out. Including the mag tube. All kinds of crap can find its way into a shotgun that spends a lot of time in a squad.
