Player Piano Company's #361 nipple reduces the size from 3/16" to
5/32".  My catalog also shows it as being an elbow.

If all else fails, I have a "fix" for a straight reducer; but an elbow
is not as simple.  For a straight reducing nipple, I have made them by
buying the appropriate different sizes of brass tubing from my local
hobby store, cutting them, then soldering the pieces together, one
inside the other.  These two tubing sizes easily "telescope" together.

If you cut the larger diameter half as long as the smaller size, after
soldering, you will end up with 3/16" on one end and 5/32" on the other
end.  Now all you have to do is set up to make 300!

I have purchased 5/32" nipples from the old American Piano Supply in a
box of 100.  Perhaps Schaff has them also.

Pat DeWitt