C'mon, think back....it was very common for rods to have multiple weights listed like a 4/5, 5/6, etc.

Here is an excerpt from a great article on the subject by Lefty Kreh:

Let me begin by saying that rod manufacturers design rods for the average person to use under average conditions. So unfortunately, most fly fishermen use only one weight of line on their favorite rod.
Written on the rod blank or handle is a code number which indicates the line that the rod manufacturer suggests is best for most customers; i.e., 6 line. To most fly anglers, this means that they should use nothing but a 6 weight line with this rod. But to get the full potential from different fishing situations, you may want to consider using several line sizes on your rod ? perhaps varying as much as two line sizes from the one suggested on the rod.
Manufacturers know your rod may be used in a host of fishing situations, but they can?t judge your casting style and fishing skills. So when they place a recommended line number on your rod, it is implied that it?s for average fishing conditions. First, understand that you?re not going to damage a fly rod using fly line a little lighter or heavier than is recommended. Certainly, at times, the rod will fish better if different line sizes are used.
Match line weight to conditions.......


I often use multiple weight lines on the same rod, depending on conditions. Give it a try.