This chart is to be used to convert the old designations into
the newer AFTMA numbers most are familiar with.
This chart is simply a starting point, you'll probably need
to cast your silk fly line to determine which line will work
best with your rod. As with most things, variances in the
construction of different line makers, finish, age, etc. will
cause a difference in how the silk fly line will perform.
The chart appeared on the Anglers Collectables Exchange.