Doug Miller fired this volley in
:

http://milmac.com/images/TubingFittings.png

Those are not "tube fittings". They're mechanical adaptors that are
probably custom-made by the company making the chair/table/tent/whatever
into which they assemble.

They are called "plastic aluminum tube joiners".

What were they part of? The manufacturer of the original device is
probably the best source for them.

Lloyd