Mossberg made a LOT of .22 rifles- there are dozens of different models. If you can give us a model number, we can tell you what years they were made.
Model number changed to 46B(a) after that year
The Mossberg 46B(b) was only made in 1947-1949, when the model number changed. Excellent target rifle.
There are dozens of different models of the Mossberg 22 rifle. You did not tell us which model, and we need that to give you an answer. Sorry.
Mossberg Model 321
The mossberg model 151K was made from 1950-1951.
Made in 1976. Link at bottom of page to Mossberg .22 rifle info.
From what I can find out it was made by Mossberg and is the same as the Mossberg model 20 and 21.
Mossberg Model 353
Egunparts has some.
Mossberg (402 Palomino)
Your Western Field model 45B .22cal rifle was produced by Mossberg and sons.The Mossberg model 26C is the same rifle.Mossberg produced the Model 26C between the years 1938-1941.
This model Cross references over to a Mossberg model 42 or 42A extractors can be purchased at http://www.wisnersinc.com/rifles/mossberg/mossBArf.htm for around $12, Vintage Mossberg .22 rifle information can be found at home.epix.net/~damguy