Other than the obvious ones plastic, with "J" lock and ones aluminum and no lock. What if any other differences are there? Will the current plastic units fit and function on older R870's?

There are no other differences.
All 870 Trigger groups will interchange.  
Remington has made remarkably few changes over the years, and a 1950 870 and a new 2006 will have almost total parts interchangeability.

The major change over the years was the Flex-tab conversion, and these will still interchange.

I know you said all, but....

Sorry for the large pics.

Is there a difference between the 20 and 12 guage trigger groups?  This is why I am wanting to know. As you can see from the pics there are three TGs. The two on the outer sides are aluminum. They are also from a 12 ga R870.The one in the middle is the synthetic. It was said to come from a 12 ga R870 as well. I aquired this from an outside source. Now other than the larger hammer, and the dip on the front edge the synthetic looks to be the same.  Now heres the problem. When the synthetic unit is placed into any of my  current 870's The action will not lock closed, like it should when loading a live round.  Something somewhere has changed. Either that or I need to go get Dr. Dremel Anyone else experiencing this phenominon? I'm lost.

I am running a brand new plastic trigger group in my 1964 Wingmaster.

Had no problems with fit. It just slid right in like it was made for it.

I "think" you may have a 20 gauge synthetic unit.
Notice that the slide lock is also higher.  That's the "lever" that's sticking up beside the shell lifter.

When the action is cocked, the lock moves up and locks the bolt shut.  When the trigger is pulled, the lock drops down freeing the slide and bolt to operate.

What appears to be happening is, the lever is up too high and is failing to lock the slide.

Also, in case I'm wrong, check the position of the REAR of the slide lock where it interfaces with the trigger extension on the left side of the group.
Notice in picture #3 how the trigger extension is ABOVE the slide lock extension.

Also notice in picture #4 how the slide lock release finger lever is farther forward then the other two.

Best guess:  A 20 gauge trigger plate assembly.

DO NOT attempt to modify the unit to work with a 12 gauge.  This usually causes trouble.

dfariswheel,
Once again you da man.
I will have access to a 20 ga later on today and will check to see if it is the same.



thanks