Gordon, have a look at Dexion Speedframe. Pretty sure that used metal connectors that fitted into the tubes with plastic inserts. For "temporary" structures you used to just knock them together with a mallet, but for more permanent set-ups it advised drilling and putting a fastener through to pin everything in place.
Keith
Also just found this link which might be what you're after, steel core, nylon coated.
Edited By Keith Long on 12/04/2016 16:45:11