The Pennsylvania State Police were doing the same about 10 years ago: going back 50 years and more reviewing old arrest records. If someone had been arrested and charged with a crime that was punishable with at least a 1 year prison term, they assumed that person was convicted, and was a convicted felon.
It was up to the individual to hire legal representation to prove that although they were arrested, they were never convicted. Arresting officer(s) never showed up to testify, grand jury did not indict, went to trial and was thrown out or found not guilty.
Only after furnishing them with proof that the person was not a convicted felon, could they get their right to purchase a firearm, handgun, shotgun or rifle re-instated.
Going thru the legal steps to have an old arrest record expunged is the only way to get an old arrest off your record.