Mark, two of the three steel stampings are the two halves of the outer shell of the muffler. I'll call them the back (which has the hole for the exhaust port near the bottom, and also has the rectangular exit port at the top left) and the front cover (which has no external holes in it). The third, flat piece sandwiched between the back and the front cover is the baffle. The exhaust gas enters through the single large round hole at the bottom, which is clamped across the exhaust port. On the inside of the muffler, the perimeter of that hole is pressed against the matching hole in the baffle, and probably welded to it. The exhaust gas that enters the muffler is then on the wrong side of the baffle: there is no external port in that side of the muffler, because there is no hole in the front cover. To reach the only exit (the elongated exit hole at the side of the top of the back pressing) the exhaust gas has to go through the small holes in the baffle. On the standard muffler, those holes are positioned as far away from the exit hole as possible, so the gas will have to bounce around a lot before it leaves. The bouncing around breaks up its pressure pulses (which are sound waves).
On the high performance muffler, a whole bunch of additional holes have been punched or drilled right near the exit, so the gas doesn't have to bounce around much getting there.
Back pressure is a crude measure of how restrictive a muffler is. If you have a simple muffler - say an old jam tin stuffed with steel wool - the back pressure will increase as you stuff more steel wool in there, and the exhaust sound will get quieter as well. More sophisticated mufflers use "interference effects", not back pressure, to damp sound waves. The Victa muffler effectively consists of two expansion chambers, with a restrictive damper (the baffle) in between them. Simplistically, if you make the expansion chambers bigger, you can use a less restrictive damper and get the same sound level. In practice you usually find tricky ways to make the expansion chambers effectively bigger without making them physically bigger, because of the space requirements. If you crawl under a dual exhaust V8 from the 1960s or 70s, you are pretty sure to find a very short "balance pipe" approximately at the transmission support mount, joining the two pipes (which come close to each other at that point). That way, each of the two exhaust systems is able to use the whole of the other exhaust system as an expansion chamber. (It only works because there cannot be two pulses arriving at the balance pipe from opposite ends at the same time, since the engine fires each cylinder at a different time from all the other cylinders). The system has to be tuned to work properly: the engine's output power changes slightly as you vary the position and length of the balance pipe.
Designing exhaust systems is an interesting blend of science, experience, and experimentation.
