As stated, the Mark X is a European commercially manufactured Mauser 98 style action. They were available as bare actions, barrelled actions and several grades of complete rifles. Yours has "proof" marks on the barrel which indicates it was sold as a barrelled action or complete rifle. A "built" rifle might have the caliber and maker stamped somewhere, but wouldn't have proof marks unless it was custom built at a major maker.
I've had a few over the years and have shot several. They are rock solid, capable of excellent accuracy, and all repair parts are still available, should you ever need them.
Congrats on a family treasure.