I've just been given a Precision cane fly rod (Tudor Works, Redditch ) its just over 9 ft, it also came with a 3 1/4" Condex reel & probably the original fly line, ....it was a fishing prize from the early 50s,

It has the makers name & address on the rod but not a hint to what line weight it would take.

What I'd like to know is, how did they know what line to use?

I have often wondered myself! I have sold two similar rods recently. One had a little teeny weeny 6 on it so that one was easy. The other said nothing. I still have one with no info. It is 7'6" or 8' depending on which top I use and seems to like a 3wt.

9' you say...well I would try a 5 line myself. Don't think you will be far out. Have to be careful not to overload them. Borrow a few lines 4 5 6 and have a chuck.