I have an old Heddon fiberglass fly rod my dad bought I think in the early to mid 1960s.                              On my rod the line weight is marked "H". A coworker is a fly rod guy, and has a newer rod. The line weight on his rod is marked as "6". I've made a bet with him.                                                          He claims that fly rods were marked with a letter for line weight well into the 1980s. I say he's wrong. I say the letter for line weight stopped by 1970, and changed to a number system.                   I really don't know a lot about fly rod fishing . For the fly fisherman here, what's the correct answer? 

Neither answer is correct.

Silk lines were sized by diameter and a ( or a series of) letter(s). synthetic lines are sized by weight and a number. So numbers go back to the first synthetic lines, and letters go through the last silk lines. There are still some "purists" who use cane rods with natural lines, but those are mostly sized by weight, and more often trial and error. Bass fished in the Catskills in the early '70s, and you needed to know your trout just for the street cred, 

Fly line changed in 1955.

Tom 

the H is only part of the original line marking.  

The line rating was either HEH (now DT5), HDH (DT6) or HCH (DT7)

 

The latest rod I have marked HDH for 6-wt is Orvis Fullflex A glass, bought new in 1979.  

 

The earliest rod I have marked #6 is a Phillipson made for LL Bean, c. 1971, so there's clearly some overlap in the conventions.  

 

Nice photo here of a Phillipson-made Orvis Fullflex - right in the middle of my time brackets above, marked HDG (6) for WF6

and if you ever get a shot at buying any 7' Phillipson, don't pass go

13 hours ago, bulldog1935 said:

the H is only part of the original line marking.  

The line rating was either HEH (now DT5), HDH (DT6) or HCH (DT7)

 

The latest rod I have marked HDH for 6-wt is Orvis Fullflex A glass, bought new in 1979.  

 

The earliest rod I have marked #6 is a Phillipson made for LL Bean, c. 1971, so there's clearly some overlap in the conventions.  

 

Nice photo here of a Phillipson-made Orvis Fullflex - right in the middle of my time brackets above, marked HDG (6) for WF6

and if you ever get a shot at buying any 7' Phillipson, don't pass go

Wow!  Very nice.