Aspirated P or voiceless bilabial fricative (/ɸ/, like the "F" in Fuji)? It's entirely likely that what we're seeing is the remnant of a transitional form between the ancient ancient Greek /pʰ/ and the modern /f/, and since the voiceless bilabial fricative doesn't exist in Latin (or French or English, old or modern) it would likely be heard and rendered as an F. More @Wikipedia
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