Perhaps these remarks will also help. As a general rule, fly fishers in the heyday of bamboo were very much like we are: big rods for big fish and big lines; little rods for little fish and little lines. If you've got a 6, 6.5, or 7-footer, the odds are they are built for 4-weight lines although the 7 could be a 5-weight. Rods 7.5 to 8-feet were usually 5's ... in fact, the standard for the day was an 8-foot, 5-weight.
Longer bamboos, to me, usually feel a bit clumsy if they are made for the heavier weights -- say 7's or 8's. Keep in mind that the difference between a 5 and 6-weight line wasn't easy to measure since standards, as such, really didn't exist.
Another point -- Inspect the tip-top very carefully. When bamboo breaks, it usually is within 3-inches of the tip. The rod is still fully fishable but the wwight line it will cast usually goes up a notch. I have one -- originally 6.3-feet, it was a 6-foot when I got it. It was still of value so I had it professionally restored. With a 4-weight line, it is a delight to fish.
Bamboo can be tricky ... You might also try this ... on a carpet place the tip of rod against the carpet and cause the rod to bend. The more symmetric the parabola, the more likely the rod is designed to throw a lighter line.
Finally, a war story ... I had wanted nice bamboor in 5 or 6-weight. Finally found a Orvis Madison, 7.5-footer rated for HDG lines although there was no way to tell whether the rating was for nylon or silk -- since it had a "6" behind HDG, I made a very bad assumption I assumed silk. When the rod arrived I "rushed" to cast it. I rigged a new Bonefish 6-weight and hurried out to my casting grass. Moments later, I damned near broke the rod. It couldn't handle a 6-weight line for long casting. Had I not stopped the rod as I went into the forward cast, there isn't a doubt in my military mind that wouldn't have broken. To this day, I either use a Cortland Sylk 5-weight or anyone else's 4-weight.
If I can help in any way, I will...