Your Remington 81 will have a date code on the left side of the barrel near where it meets the receiver. It will be a single letter followed by another single letter or pair of like letters.
The Model 81 was factory drilled/tapped for an aftermarket tang sight on the rear hump of the receiver. Sights were made by both Marble's and Lyman, but neither company still catalogs that model. In fact, expect to pay rather handsomely if you locate one. Persistent Google searches and auction searches on the gun auctions and even eBay are probably your best bet on the sight.
The autonomic nervous system provides for involuntary muscle function - the work of breathing, digestion, and so forth. On some folks, that's a pure waste of ingenuity.