I fear I am repeating myself: I am 100% sure this is an Original Mauser pre-1912 commercial sporting rifle, albeit of an unusual, rare configuration. At that time Mauser experimented with several foreend lengths to satisfy customer's tastes. You will find the complete serial number on the left side of the receiver ring, the rear of the magazine box and in the channel under the barrel shank. Also note: The set trigger assembly is not mounted in a seperate housing mounted in the trigger guard like all aftermarket DSTs are. Instead, the trigger guard itself is milled to take the set trigger parts with the long spring on the right side.
May you please post, in due time, a close-up photo of the receiver bridge and the peep sight?