You have picked yourself up one very fine rifle and at a very good price to boot. As a fan of old Mossies I will tell you that you've got yourself the top model of their semi-auto .22 rifles. The 151M was introduced in 1946 and was an improvement over the Model 51M which was a pre-war model. The 151M was followed in 1947 by the 151M(a), the 151M(b) in 1948, and the 151M(c) in 1954. Your rifle should have left the factory with an S130 receiver sight which will serve you very well. You may be interested in knowing that the 151M(b) was originally offered at a price of $30.90. (By the way: That full length stock is actually a 2 piece. The junction is under that steel strap near the muzzle.)
Congratulations on a great deal on a great old rifle.