Walter;
The 19/20 or 20/21 you see at Schoff tackle are measurements for metall fiberglass rod ferrules. Think about the taper of a fiberglass rod, you need the male to be 1/64ths smaller than the femaole to account for the taper in the rod because you can't turn it down so the ferrule fits, the wall of the rod isn't thick enough. The female on those kind of ferrules is swaged large enough to fit the male. I missed what kind of rod you're working on, if fiberglass, that's what you want, if bamboo, then you need a different and more expensive ferrule. If you want to replace what was there, get a dial caliper and measure where the bamboo was turned for the ferrule, then call Rick's Rods in Denver and tell him what you have, he has thousands of NOS ferrules for all kinds of rods, he'll have what you need.
john