The tubes are 1" and 3/4" square 1/16" walled aluminum tubing, 4" long
The two 3/4" tubes are side by side with 1/2" holes (two of them) connecting them
This was done to increase the chamber size of the muffler as well as make the assembly more rigid

The 1" side tubes have 3/4" x 1 1/2" square holes in the side to accept the 3/4" tubes
These were done by drilling a series of 1/4" holes to remove the bulk of material and then enlarging the hole with a coarse file
The fit of these holes is very tight

The 1/16" gap between the tubes (inside the side tubes) is filled with 1/16" fiber gasket material
This is shown in gray in the diagram
These are inserted from the end of the side tubes, prior to assembly

The whole thing is held together with long bolts and locktite
Not shown in the diagram but mentioned in the previous post is the 5/8" x 1/4" hollow spacers around the mounting bolts which support the muffler when you tighten the bolts that hold the muffler to the engine.

I hope this is clear

Pete