Thanks for all of the replies and information! I haven't found the exact brand/model online but, given the history of finds from this park, its found depth/location, and lack of extensive oxidation, my guess is mid-twentieth century.
Looks like a Lyman .357" (.38 Caliber) 160 gr - from the 1930's to still in use.
I shoot muzzleloaders and dig back out and recast the lead from the .530" and .648" round balls (handy to have a metal detector for that). It only takes five or six years for a thick, white oxidation coating to form on them in my local NY soil. Bullets also tend to bury themselves when fired, so depth can be deceiving.