As posted above:

1941, 235

Group No.: 3.701
Part Number: 3656598
1940-1949 Commercial, Utility Muffler
http://chevy.oldcarmanualproject.com/parts/1929_54/195.htm

Group No.: 3.701
Part Number: 3693710
http://chevy.oldcarmanualproject.com/1929_57chevyparts/57cmpc0281.htm

I do not know why it changed in 1950, but after seeing Paul's muffler, one noticeable difference (in addition to dimensions posted above) is (I think) that the 1950-56 muffler has the input and output pipe-fittings in the middle of the muffler.