Was there a time when it wasn't widely accepted as PA? At Gettysburg today, I saw some old plaques that reference Pennsylvania as PV. If it was originally PV, when and why did it change?
The current state abbreviations were officially introduced in 1963 with creation of the 5-digit ZIP Code system. Before that, USPS policy was that both both city and state names were to be written out in full. However, the Table of the post offices of the United States from 1831 shows that "Pa." was used as an abbreviation for Pennsylvania by that date by at least some groups. I've also seen old documents that abbreviate Pennsylvania as "Penna." but never as "PV".
Perhaps "PV" was meant to stand for "Pennsylvania Volunteers" instead? That would fit with the naming convention for some civil war regiments. The 11th Pennsylvania Cavalry, for instance, was also known as the 108th Volunteers. I'm not entirely sure why this naming convention existed though.