I should mention that being "cast pewter" does NOT mean that a button is a genuine Original of a historical button. Many cast-pewter Reproductions have been made. I myself own a cast pewter Reproduction of this type of button, made and sold for use on battle re-enactor uniforms. Unfortunately, Reproduction solid-cast buttons are frequently sold on Ebay as being Original ones ...so, collectors should be wary.
If I may ask... how did this button come into your possession?
Your button appears to be a US Revolutionary War era Continenal Army button, made of dolid-cast pewter. Collectors refer to it as a RevWar "intertwined USA" button. Several variations of that type are known to exist, being primarily variations in the shape of the letters, and the form of border surrounding the letters. See the book "Record of American Uniform and Historical Buttons, With Supplement" (by Alpheaus H. Albert) to view photos of the different varieties of "Intertwined-USA" buttons. The only near-exact match for your "borderless" Intertwined-USA button is shown in that book as button number GI-2-NY. It has the letters NY below the intertwined USA. But I cannot see NY in your photo of your somewhat-corroded button's face ...but you can examine it closely in real-life. Does it have the letters NY below the USA?
I should also mention that your button does have the type of solid-cast loop which is correct for pewter Intertwined-USA buttons.
I suggest you contact Mr. Don Troiani, who is a major collector and scholar of American Revolutionary War buttons. He is the best guy to ask about your button. If he feels your button is an Original one, and he if he doesn't have an example of it in his collection, he might be interested in buying it. Please let us know what he says.