H&R is the best known current maker of single-shots. They do show one model of single-shot 20 with screw-in chokes...the
Topper Deluxe Classic. Not sure how difficult it would be to actually find one, though. I recall hearing the Topper line was being discontinued...though they're still showing Toppers on their website.
http://www.hr1871.com/Firearms/Shotguns/topper.asp