About 2008 I issued a request for 16g choke measurements and a number of respondents provided data for Parker, Fox, Baker, Ithaca, and Smith guns. Almost all had a choke length of 1 1/2 to 2 inches with a very short parallel section at the muzzle.
Unfortunately, I have no data for 20g
P. 394 of L.C. Smith "The Legend Lives" shows the 12g choke specifications; a 1 9/16" taper segment with a 1" parallel segment.
I would certainly request a Research Letter as factory original 24" 20g guns have been documented