Bob Elliot guesstimates that only about 6% of total Lefever production was in 20ga. Of these the majority were in DS grade, which had steel barels. This in & of itself can account for the scarcity of Damascus bbl'd 20's. But to directly answer your question, yes they were built, several are listed in the LACA SN list. The 20ga was cataloged at least as early as 1889 at which time the only mention of steel bbls was Whitworth ones on the optimus grade. It is interesting to note, though, in the 1913 catalog the H grade is listed as being available in Twist or Carmen Steel in 12 & 16 ga's but only Carmen Steel in the 20ga. Thus to further your question, Has anyone seen a Twist bbl'd H grade 20ga??
