Nckr624:
Many folks talk about underlining old Orvis rods as if it's a given. I see it most often with eBay sellers hoping to broaden the appeal of their offerings to today's line shy buyers or an attempt at turning the rod into something it never was, and many times never will be.
I own more than a few Orvis bamboo rods from the 1940's through the 1970's including 3wts to 7 weights with a bunch of spinning rods thrown in. I fish every single one happily with the line weight written on the blank and I am far from alone in this practice.
Also keep in mind there WERE plastic fly lines in the 1960's and quite possibly your great uncle used one on this rod and it may have been a 7wt. Orvis continued using the old "letter" line designations for a LONG time, later adding the modern AFTMA line weight equivalents to the blank markings. Whether or not you feel they match up is not an excuse except if you DON'T like what you feel.
It's really no different than comparing what several different manufacturer's call a 7wt or 6wt line today.
Like others, my suggestion to you is to try both line weights and see what you like and don't be afraid if it ends up it is a 7 weight. To this I'll add to STAY away from any hybrid tapers or mislabeled (meaning heavy) fly lines like a Rio Gold which will throw off your assessment.
Have fun!