I have a Remington 870 Express I bought in 1989. I always read about locking safetys, divits in the magazine tube,and plastic trigger plates? When did they add these features to the 870? My 870 has none of these things thank God.

I have an 870 express magnum I bought in 92 and it dosent have any of them so probably in response to the AWB or some crime bill around 94 I think the divots are a way of trying to prevent people from adding extension tubes to the mag but the plastic trigger plates are just a money saving manufacturing step.
well actually there are two "dimples" or diviots at the end of the mag tube on mine to hold the spring/follower cap in when you remove the mag tube cap but that just keeps the spring from flying across the room or into your eye.

My 870 just had a removeable washer type cap that holds the spring in.

I have a 1996 Express 870HD and it has the dimples but the trigger group is all metal. The locking safety is a very recent addition. It would also be the very first thing I would remove from a new one if I were to buy one.

Those dimples are a pothetic attempt at keeping people from adding a mag extension.  I think they were just an easier way to keep the spring cap/spring in the tube.  I wanted an 8 shot extension on my 870 Super Mag that I got 2 years ago.  I just drilled the divots out and screwed a standard 870 extension on.  
7- 3.5" 00 Buck MUHAHAHAHAHA