I often shoot a M21 skeet with the usual Sk1 and Sk2 chokes. I love it for the usual sporting clays layout. The Winchester specification sheets for chokes show that Sk2 is supposed to be improved cylinder which is about 0.005" constriction but can be as much 0.009" depending on bore size. Mine is 0.011" which is about the top of their range for modified which is 0.008" to 0.012". In 12 gauge Sk1 was changed in 1963 to straight cylinder. Before that, there was a pinch in from the bore diameter about 2" from the muzzle by about 0.005". In the last two inches the bore flared out to about 0.020" over bore diameter. One book on the M21 indicates in a table that the probable reason was to give a hotter pattern center but about the same spread as a cylinder bore. Botton line. I think you will enjoy the cylinder/modified combination. Have fun!!
