[utopia]

Now that you mention it, I think I must have seen the "twangy" 2-stroke exhaust stubs in action .. possibly in a paddock somewhere, in the 70s, or maybe at a trials event.

And .. funny how you forget the most obvious stuff sometimes .. I once had occasion to test the performance of "washer-form" gaskets made out of alternate, concentric wraps of thin stainless foil and an asbestos-like material. The foil had a wibble in it too, which gave the gasket springiness.
They might be a tad softer.
As I type, I'm trying to summon a trade name. I do remember their works being in Heckmondwike .. well you would, wouldn't you.

Yes, I thought the MADASL systems used the spring method. Dukedesmo's is one of theirs, I believe.
And from your description, my Sil Motor system is exactly the same.

The Sil headers measure up at about 42.5mm bore, 44.5mm OD, so the wall thickness is only 1mm.

Twiddling with the Sil system, a realised that I didn't much rate the fact that there are only two springs at the header joints.
Even clamping pressure all round would seem to need the three legged stool principle to be considered .. ie a third spring.
But that would be too messy.
And then again, there are only two studs anyway .. but nevertheless I feel that either clamps or a third spring would seem adequate whereas just the two springs seems intuitively borderline.
On top of that, the springs on the front header of the Sil system both go to the same side of the anchor ring thingy .. which seems unnecessarily lop-sided.

350 Yamsel, eh ?
That makes the pulse quicken.

ps.
Maybe the ooomph in a 2-stroke exhaust is not so powerful .. and maybe the pressure at the stubs is lower than you might expect due to the "suck" from the spannies ....?
... so to speak.