Congratulations on a great find. My big Rigby is a .375 H&H on a standard 98 Mauser action. It is #6189, 22 numbers before yours. It was ordered in mid 1969 by a shooting friend who was the CIA station chief in Kabul at the time he ordered and received the gun. Like yours, it doesn't seem to have been carried much. I have correspondence of my friend's effort to arrange tiger hunts, which, at the time, were about closed off. My gun was ordered with and received with a very Griffin and Howe style quick detachable side mount and express sights. He received the gun in mid 1970.
