I have Understood that there has never been a Regular frame 20 gauge ever located.
From the FAQ: The Featherweight (FW) frame was introduced with the 20 gauge gun in 1907 (SN 1104 completed December 4, 1907), and was produced in two sizes: 12 gauge and 16/20 gauge. It has not been confirmed if 20 gauge guns were ever built on Regular frames.
With the research out of the book Mr Stubbendeck wrote it appears that there is documentation of some being built. Has any members turned up a Regular framed 20 gauge yet?