The gunmakers of Dera could duplicate Belgian proof marks quite easily. And I can see how they might be applied to a fake German rifle as decoration. I remember seeing a gunsmith busy making a punch with a Persian Lion; it would be applied to a fake Persian contract Mauser he was building.

For what it's worth, Dera Adam Khel, or Darra Adam Khel is the village near Peshawar famous for gunmaking. Even though it isn't in the Khyber Pass, it is the village people are talking about when they mention gunmakers of the Khyber. When I was there in 1974, probably 80% of the shops in town were gun shops. The remainder were tea shops or opium shops.