Hank, I am going out on a limb a little, but I have found that back in the day, when Lyman changed things they tended to keep some of the things the same from model to model to save on changing tooling and parts. You might want to try a drop tube from a # 55 powder measure and see if it fits. From looking at some of the pics on line, I am feeling that it just might. 
Sam

Hank45,
Drop tubes from a Lyman 5 or 6 are identical, their diameter is exactly 1/2 inch. A drop tube from a Lyman 55 measures .558, so will not work.
Busted Lyman 5s for parts sometimes show up on eBay and might be an answer. The thing to know about the 5/6 drop tube is that it is internally tapered at both ends, short taper at cartridge end and long taper from measure end to prevent powder bridging. You might also note that Belding and Mull powder measures are also exactly 1/2 inch, as are any number of easily available adjustable brass muzzle loading powder measures; might be an interim solution. Alternately a machinist could make short work of one and at the same time make a 7 inch or longer drop tube as well. 
Bob