I suspicion that the Reservoir Park recipe is the famous Cornell Chicken recipe. The recipe was made known to me when I worked at Ag Sci at Penn State. From the Cornell site (see hyperlink below) the timeframe seems right as it was “discovered” in 1950 and probably made its way through the East first and foremost
CORNELL CHICKEN
1 egg
1 cup vegetable oil
2 cups cider vinegar
3 tablespoons salt
1 tablespoon poultry seasoning
1 teaspoon ground black pepper
Crack the egg into a medium bowl and whisk until beaten. Slowly whisk in the oil until fully blended. Then whisk in the vinegar, salt, poultry seasoning, and ground black pepper. Set some of the sauce aside to use for basting while grilling. Place chicken in shallow baking dish, and coat with sauce. Cover, and marinate in the refrigerator for 24 hours.
The pamphet from the website shows a bbq pit, etc. If you do this in a Webber kettle or gas grill the key is a good, low fire since the chicken takes awhile to cook through and you baste throughout.
With the oil in the basting sauce you get some spectacular flare ups which is highly reminiscent of the pits at Reservoir Park!!!