I'd be surprised if they had registration of births, marriages and deaths, as we know it, at that time in Pennsylvania. The start dates seem to vary from state to state, often from surprisingly late on.
In the US they only seem to use those darned initials for their middle names! I sometimes wonder if they even knew what they stood for themselves. So even if John had signed that will, it would probably have been as John L Jones!
Your best hope would be if John and Mary married before they arrived in America
This might be the lady named in John's will to be guardian of granddaughter Mary - Ann Morgan the wife of William L Morgan. In Sharon, Mercer, in 1900. Both were from England. Can't find their marriage either (which was probably in America?)
http://www.rootschat.com/links/01i23/ John
