Pulled 3 starters at the junkyard to replace one that has a bad solenoid.
Have a 2002 Buick Regal LS with 3800 series II engine.
Found some on 3100 engines and one from a 3800.
My starter has about 10 teeth on the gear that engages the flywheel. Some of the ones I pulled had smaller gears with only about 8 teeth.

Why the difference?

Also one GM Delco starter was slightly longer (about 1/2 inch) than mine and had a small black hollow tube about 1 inch long at the rear of the starter and at the bottom. It fell off when I touched it. Looks like it might be a drain hole in case some water gets in to the starter. What is this really for?

3800 uses 11 teeth 3100 uses 9 if both flywheels are same number of teeth the 11 would crank faster from the gearing.The super charged 3800 cam with a 1.7 kv starter then non came with a 1.3kv starter so one longer as you found.

Now if the 3100 has more teeth on the flywheel it would be slower cranking but have more toque cranking.
I do believe the 1.7kv starters have the bolts to the housing on the outside
to offer more support on the case.

As far as the tube it would be a water drain.
Like you thought

As far as solenoids
here is a NEW one we use all the time for $13.68 with free shipping!!
it just has one extra wire hook up you don't use.
Sad nowadays you cant go to a auto supply and get
a starter solenoid.
They just want sell you the whole starter with solenoid for $140 plus.
And I came up with starter drives and brush assembly'* to if any one needs those
too.
New Mercury Marine Solenoid 8 Cyl Engines 90 01 PG260F1 PG260F2 PG260M 1114583 | eBay

Thanks for the info. Got a similar starter with I guess 11 teeth and also another solenoid at the junk yard. Used the solenoid on the original starter. It works fine and started the car right up. Now I have a spare starter.
I used to rebuild starters, brushes, solenoids and even starter gears. Almost impossible to find the parts today except maybe online and maybe in a junk yard.