I dug the button in a park in Berks county, Pennsylvania. I believe the park has been there since the 40's, and before that it was just corn fields. I dont know how much space you need for a reenactment, but the park is pretty big. It has three ball fields, three soccer fields, and a creek running through the middle of it. So I think its entirely possible there could have been reenactments there.
Only takes one person to lose one button.
Being found in a park opens many possibilities. A Fourth of July Celebration, Bicentennial Event in 1976, Historical Theme Picnic, etc.. Just one person dressed in historical depiction of a Revolutionary War soldier, could lose a single button. Or even plausible as a lost item in a park by a youngster, carrying a reproduced momento purchased or acquired elsewhere.
I've looked through all the examples of Continental Army pewter USA buttons shown in the reference book authored by Don Troiani. There are no actual matches that I can see, to the precise details noted on your example from the park in Berks County, PA. Look forward to any additional information you may gather, through direct correspondence.
Context of finds from sites has a lot of bearing. However, there are many unusual circumstances that lead to what is found, and where.
For many years, I wore a reproduction Civil War leather belt and CS buckle while relic hunting. The buckle was a stamped brass oval CS, with solder filled back, and arrowhead hooks (unlike the original CS buckles of the period). Knowledgeable diggers and collectors would of course immediately recognize the details on this particular CS buckle as entirely a relatively modern reproduction item. However, I often laugh about the possibilities surrounding a potential loss of my buckle, in a remote area holding mid-19th Century relics. This in fact nearly happened more than a decade ago, as I was on an uninhabited hillside relic hunting, where once many structures had stood in the 1860's. As luck would have it, the main hook on my 20th Century reproduction CS buckle broke entirely off, and my belt would no longer support my pants and host of other gear around my waist. Being a rather long walk back to the vehicle, and not wishing to lose valuable searching time, I quickly improvised a "field repair". Since the area was strewn liberally with square nails, and the soil was rather kind to preserve these quite well, I visually searched about for the perfect shape and size nail to replace the broken hook on my buckle. Soon finding a likely candidate, I took off my belt buckle, placed it on the ground, and proceeded to use a rock in hammer fashion to pound a square nail through the face of the buckle, creating a hook on the back for utilitarian use. Since the brass face on solder filled buckles is rather thin, and the solder backing soft metal, the nail punched through quite easily. Soon I was back in action, with my belt and pants secured tight. Quite pleased with my ingenious repair, I was back digging relics in a matter of minutes.
Often though, I smile when contemplating the thought of possibly losing that repaired CS buckle, with an original 19th Century square nail punched through, in some remote 1860's period site. After a few years in the ground for patina, some future relic hunter would be in for quite a shock, when finding my buckle. Thankfully, I have since relegated that repaired buckle to a drawer in my home, and saved the thrill and heartbreak from future generations of relic hunters.

Imagine a CS buckle, found on steep hillside far from any populated area, where no reenactments have ever taken place, among the remains of 19th Century sites, with a rusty square nail imbedded clean through. There would be almost no logical explanation other than to deem this a "period" find!
CC Hunter