All of this information is gleaned from Jon Speed's excellent "Mauser Small Bores" (1998):
Your rifle is surely the ES 350 B model.
If so, it is believed that serial # 174223 was ordered in July of 1938 and that serial # 183,997 was ordered in September of 1938, so your rifle was made in that approximate time frame. Factory data does not exist for these rifle, so all that we know is gleaned from the few people who actually know when their rifle was ordered. Also, Mauser was listing several models of 22's in this serial number sequence, so this is total 22 production, not just ES 350 B's.
Crown proofs indicate manufacture before 1940, when Nazi eagle proofs were adopted in Germany.
The number 459 is the Mauser Obendorf inspector's number that is found on most Mauser 22's at this time.
The crown over G is a barrel inspection proof.
The crown over U and B are firing proof mark's, but I am not sure why there are two.
The groove on the top with the numbers is for a adjustable tangent rear sight. There may be an N in this sequence for the "Normal" sight position. I take it that you are missing the sight, which will be hard to find.
The Ch 51 indicates Chrome 51, the barrel steel.
This rifle was Mauser Obendorf's top quality single shot 22 target rifle of the time and cost more than America's beloved Model 52 Winchester. You have an extremely nice 22 in this one, so enjoy it.
