Here's a close up from the previous photo.

still looks like a cocking indicator with a washer as a bearing surface, wouldn't you love to know the how and why that got that way.

as for the seller- its his job to answer your questions, especially when there is an oddity with the gun. If you cannot get hands on - ask him to un-cock and re-cock it to see if the screw slot rotates 90 degrees.

It's just my opinion, but unless this mod can be documented to Parker or Remington, it's just an oddity that doesn't really add to the value. The quality of the workmanship is what will determine if it actually detracts from the value. If it's only marginally well done, it may have the effect of a carriage bolt through the head of the stock to fix a crack.
Most rotating cocking indicators I have seen on English or European guns have their slot fillet with gold.

I think that when the gun was restocked, they added a bit of metal that extends to the rear to stop the stock from spreading. Inletted in under the new stock. Probably to prevent the damage that occurred to the original.

How about that gun spent part of its life as an A.P. Curtis try-gun, and was then restocked in a normal fashon?

Definitely needs a research letter.

Researcher, Curtis try guns have no extra holes. Maybe a Parker try gun.

I'm in Kodiak, and I seem to remember some style of try-gun with a screw in that location. Maybe not an A.P. Curtis.

I think the original Parker try guns have a cut in that location.

On Parker try-guns wasn't there a boss and screw that projected into the stock from the rear of the frame?