The script is Devanagari script, which makes me think this could be linked to the Buddhist era in Bamiyan and other parts of Afghanistan. It could also be a later piece that ended up in Bamiyan through trade. Sanskrit is one of the many languages written in Devanagari script, and Sanskrit is used for Buddhist texts. For verification of the age it is best to ask a museum to take a look at it.
It may very well just be a sign saying "Ladies loo to the right" ! Id email some professors to see if I could get it translated.