This double-denomination error is certainly authentic. Whether the cent was minted in Philadelphia is less certain. There are a few double-denomination errors in which the two strikes were from different mints. Most of these are considered to be assisted errors. One can imagine a mischievous employee at the contractor that provides finished cent planchets to the Mint tossing in a struck coin. However, absence of a mintmark on the host coin doesn't necessarily mean the cent was struck in Philly. The mintmark could have been erased by the nickel strike. Or the mintmark could have been weak due to grease clogging the corresponding recess in the cent die. The area needs to be inspected under a microscope. If the host coin was truly missing the mintmark, then this error is worth well in excess of $1500. I'd guess anywhere between $3500 and $6000.
Error coin writer and researcher.