Out of curiosity (not related to actually doing any plumbing):
I came across a 1.5" diameter, 6" long metal "extension tube" in the
plumbing section today, next to the brass sink tailpieces/couplers/
traps. The outside of each end was threaded (fine pitch), maybe 3/8"
worth. The neighboring 1.5" metal accessories all had coarse
threading...I couldn't find anything to mate with the fine threads.
Nor could the hired help...he also had no idea what/how it was used.
Closest I could find online was a "threaded tailpiece", but that still
doesn't explain what it threads into.
Anyone have any ideas?