The letter reffers to the Gauge of the Set.
I just checked with a RS665LD with is 45-130 Long Scale.
They fit my Lakland 55-02 35" Scale through the bridge, not through the body. The silk is barely past the nut, but they work.
Now if you have the strings on hand... why not just open the pack and check for yourself.
Long Scale is 34"
Extended Scale 35" and up
Medium Scale 32" and Short anything less around 30"