Never heard of a model 50, model 51 yes, but, no model 50.
There is a model 50. It is a like the 51 but without the mannlicher stock.
http://home.epix.net/~damguy/exploded.htm
It was also made for Wards Western Field as the model 04m-489a
The 51 has an S100 receiver peep sight as does the 51m (full Mannlicher style stock). The rear sight on the model 50 has the barrel mounted one adjustable for windage and elevation. Check out: http://home.epix.net/~damguy/models.htm
CF
The difference is explained in the name. A rim fire rifle uses ammunition where the hammer strikes the rim of the bullet. In a center fire rifle, the hammer hits the the center of the bullet. Rim fire ammunition is usually small, like a .22, and center fire ammo is generally larger.
The Mossberg 51M (no s) is a semi automatic .22 rifle, It was made from 1939-1946. It is an accurate target rifle, and has 3 sights- front, barrel mounted rear notch type, and a peep sight mounted at the back of the reciever. Values will vary based on condition, from about $75 to $250 for an excellent specimen.
22 Long Rifle
Any rifle that the firing pin strikes the bullet casing right in the center of the casing, causing it to discharge is called a "Center Fire Rifle." In contrast a "Rim Fire Rifle" the firing pin strikes the edge or rim of the bullet casing causing it to discharge.
40-1000 USD
The 44 rim fire Henry rifle was developed by Benjamin Tyler Henry.
Rim fire, just like every other .22lr put out there.
Center fire- the firing pin is placed to strike the center of the cartridge. Rim fire- as the name implies- firing pin strikes the very edge of the rear of the cartridge- the rim.
32/22 cal rim fire short rifle history and value
They were originally made in New Haven Ct. Some are now made in South America. You can check with Mossberg Customer Service through their website.
Slows down gases.