I think "TL" first showed up on Orvis models as an improvement to their Trident rods series, which was then their top of the line. I don't know what the initials meant, but the main difference between Trident and Trident TL appears to have been more sophisticated taper design, according to their website: Graphite Fly Rod Making History | Orvis Fly Fishing Rods
. Silver Label was the name of their budget price-point series for a while. Maybe Silver Label TL was a similar improvement in taper design for their budget rods, but I'm just guessing. Of course, you could always give their toll-free fishing number a call and find out the real answer straight from the horse's mouth.