Simple plumbing fixture question

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?
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On 4/7/2011 2:00 PM, Borrall Wonnell wrote:

i have a brass tub drain assembly that matches that description. Not sure why it would need to be threaded on both ends though.
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Steve Barker
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Some of the older traps are threaded insteaed of of a slip joint connection
wrote:

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It's a "tailpiece for dummies" based on the manufacturer's attempt to save you that infamous "extra trip" to the store that every plumbing job seems to require.
You try to screw one end in and if you don't mess up the threads, you use a pipe cutter to cut off the other threads.
If you do mess up the threads, you've got another set to be more careful with.
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