I want to know if it is okay to shoot 3" shells out of my 870.  The barrel says 2 3/4" or 3", my receiver is just an 870 express but the serial number end in M.  Is it okay to shoot 3" or not?

Yes.  Your barrel and your "M" tell you so.  Didn't believe them?

As crazy as the guys in GD get sometimes, they were right.  

You're all set to go with 3" shells.

What letter will the serial number end with if the gun is able to fire 3 1/2 inch shells?

It will say Super Mag and have a sliding piece on the back end of the bolt.

This.

The receiver will say "Remington 870 Express Super Magnum" as opposed to "Express Magnum" or "Express"  

The ejection port will also be slightly larger than the 2.75"/3" receivers.

Just to add so as not to confuse new buyers.  The serial numbering system for Remington was changed by Freedom Group the new owners as they wanted a number system consistent with all the companies they own.   So now Remington shotguns have a serial number that starts with RS which simply stands for Remington Shotgun.  The suffix will be a letter but has no significance other then to indicate the date of manufacture.   This is not an issue for receivers as they only make 3" receivers and 3.5" and the 3.5" will have Super Magnum on them,  

So if the weapon serial number starts with RS you are fine with 3" magnum shells and if it has Super Magnum on it you are fine with up to 3.5" shells if you barrel is also chambered for 3.5" shells.  If you pick up a used 870 It is also important to read the barrel as you can have a 3" barrel that was installed on a Super Magnum receiver or a 2 3/4" barrel installed on a 3" receiver.  It is also very common to have a 3" barrel installed on some used older 2 3/4" receivers.   Always check before you buy a used 870.

Thanks guys.