How old is your mossberg model 800b?
The Revelation 220B cross references to a Mossberg 800B. Unfortunately, the rifle has been out of production for some time, and the usual sources of parts are empty. You may check with Havlin Sales through their website- Mr. Victor Havlin is one of the best sources of parts and info for Mossbergs, and he and his wife Cheryl run the Mossberg Collectors Assoc. Good luck-
Your rifle was made for Western Auto hardware stores by O.F. Mossberg Company, and cross references to a Mossberg Model 8008. Unfortunately, there is almost no info available on that model. That series would have been made in the 1960s-1970s. If the gun has no serial number, made prior to 1968. You can also check Mossberg Customer Service on line.
No published sn data.I can say that the Mossberg model 151 semi-auto .22cal rifle was made from 1950-1951 if it is the model 151K.The model 151M was made from 1946-1947.The model 151M(a),151m(b),and 151(c) were made from 1947-1958 and were close to the model 151M with minor changes in the buttplate,and sights.