I'm looking at getting an old used Stevens 311 for rabbit hunting. Either a 20 or .410 ga. What is the difference between the 311A, H, D, etc........? I know that they made a bazillion 311's in 12ga. But the .20 & .410's are a bit harder to find. I also know that the 311's were a good gun, but later became junk. I want to find what letter designations were for what years made, and if they meant different features.