Here's a link to the auction. It's in Northern Michigan.    John Peck Auctions

Jon

The very early production Model 1886s were equipped with a "single-set" trigger (similar to the Model 1876) when special ordered.  Most of the Model 1886s were made with the much more common "close-coupled" set trigger, often referred to (incorrectly) as a "double-set".  That "pin" is actually a slotted screw, and turning it adjusts the amount of pull weight needed to release the trigger.

Bert

Honest Jon said
Well thanks once again Bert and I just realized this should have gone under Winchester Shotguns in the forum. I have a hard time relating 1885 SS to shotguns😵‍💫!
  

The specific category for these is not really a concern, as they were a "specialty" variation of the Single Shot rifles.  Many years ago I wrote an article discussing them.

Fall 2016 (winchestercollector.org)

Bert

The small serrated tab is the Take Down locking device.

Take Down

Solid Frame

Can you provide the serial number on the gun you looked at?  Thus far, I have documented just (107) high-wall 20-ga shotguns.

Bert

I’ve seen an 1885 SS Shotgun and wondering what visual clues would indicate it as a takedown receiver. From the brief look I had, it has a small, grooved tab beneath the receiver where the barrel meets the receiver. Looks like it could be a takedown release. My encounter with this gun was so brief I don’t have much more info and certainly no photos. Not even sure it would ever even be available to me. If anything develops I’ll surely let you’all know.
Jon

steve004 said
I see the Remington-Keene is on the above list?  
  

That is correct. If you have one, or have an original conversion to 44-40 (for whatever reason)....it is a custom. I have no idea why...but it is what it is...according to Lyman's 49th. I have never seen evidence of one.

But does anyone know what the pin is hanging down by the trigger?

You're kind words are appreciated. My wife and I are very grateful, in so many ways to have him home, and enjoying the private sector in life, as so many of us can relate to. We constantly try to give back on such a small scale in comparison to others who might need the uplifting support along the way. 

Tony

Anthony #5411 said
I have been following this thread since the beginning and didn't really have anything positive to add, until it turned here at the end with oldcrankyyankee, adding his comments on the caliber that was used by the well known Sniper.
lets not forget that GySgt Carlos Hathcock won it in ’65 with a 300 win mag. Semper Fi. -oldcrankyyankee
First off Tom, Thank you for you're service! 
Let me add that my late Brother in Law, the CW05, who was a Navigator on the C-130 Transport planes, in the Corps, was able to help out and get the rifle or rifles that Carlos Hathcock used, into the Marine Corps Museum in Triangle, Virginia, near Quantico as we visited there last year, as a section was dedicated to the Air Wing my BIL was attached to! If you haven't been to this Marine Corps Museum, it's worth the visit.
My youngest Son, a Sniper in the Corps, used several rifles to qualify, but said not one M-70 was used some 15-20 years ago, when he was using the "R" word as a bolt rifle that Mike mentioned, along with a few others, but not one M-70 Rifle.  He was fortunate to meet the other well known Sniper Chuck Mawhinney, in Hawaii at the Marine Corps Base, while attending Sniper school. 
This conversation has seemed to come full circle here for me, as the Louis find at the last OGCA/WACA show, this past weekend of the M-70 wood shipping crate/box, that has the Triangle, Virginia markings, along with the "Evaluators Limited", markings that the well known, U.S. Marine Brigadier General George O. VAN Orden, and address that he is known for starting and associated with in his retirement from the Corps.
Tony
  

Tony - kudos to your son for his achievement!