I'm not sure. You got one? I've seen really nice ones go for $75, and fairly beat up sticks go for $150 plus.
I'm afraid I'd pay "Stupid Money" for a really nice one.
As for the serial numbers, all I know for certain is the series go in alphabetical order. For most of the Fenwicks I've picked up, the H thru L or M series rods go from chrome to "Carbaloy" (sp) guides. I'm not sure which series saw the shift from metal ferrule to the all fiberglass "Ferrulite", but it was early. Some of the later ceramic insert guides had a strange 'flat metal' characteristic I'm not fond of, but they are sturdy. My only real No-No is: I'm not interested in Fenwick rods made in Taiwan. I've fished several ultra light and light spinners and they are nice, functional rods. I'm looking for USA built, preferably cork handled, and again, something in the H-M series range. A good one older than "H" would probably be out of my price range......