Follow the fingers - An unusual milepost

I passed this milepost on my walk today. Royal Exchange Square is a public square right in the heart of Glasgow City Centre. I don't know how old the signpost is but I was intrigued by the spelling of Aberfoil which nowadays is written as Aberfoyle. Wikipedia revealed that "Historically, alternative spellings such as Abirfull, Aberfule, Aberfoill and Aberfoil have been recorded before the current spelling became accepted by the 20th century"