At a high level, it’s a 1940 Gustloff K98k. There may be some surprises, & more & clearer photos would help a lot. At this date, (1940) most of the metal parts, as well as the stock will show the rifle serial #, 51m. (probably not the ‘m’, but the 2 digits). There will also be other stamps on both metal & wood, indicating who made the part & which military inspector’s office accepted the part for use. All of these marks are important in ‘telling the story’ of your rifle before it fell into greatGrandfather’s hands.
take a look in the photo reference at similar rifles marked ‘337’ to see how yours compares.
