lubbockdave, a general rule with the 311 is the that the .410 choke is full and full. 12, 16, and 20 gauge 26" barells are imp. cyl. and modified. 28" and 30" are modified and full, but I think some of the 30 inchers might be full and full.
I don't know about the 311A, but on the 311H I own, the screw should definatly not touch. Your right about the serial #, all guns after 1968 will have one.