How does one know if theyve stumbled across a sports muffler in a wreckers yard? Is there a part number stamped on it? Single tip or dual tip? Etc etc
And which models was it fitted to. Am i incorrectly assuming the GTVi had one?

The gtvi had a vrx exhaust on it, which is a sports exhaust, just a mild one.

If it's aftermarket it should have a part number stamped (indented) on the top near the mounting points depending on the brand

Pretty sure the TF and TH Sports had the two small dump tips on them. The TJ Sports has the one, bigger dump tip. GTVi has the same as the TJ Sports ie single large dump tip, not the straight tip that the VRX has.

I found one in the boot of a GTVi stamped j20-06 with dual tips that point slightly down. Not sure if thats the original or someones swapped it

Edit: my KF Ei has dual tips

TH sports were the first to have the single outlet, the GTVi did not have a VRX muffler, it was the sports muffler with the droopy single tip to allow it to clear the bar. AFAIK the sports and vrx mufflers only had different tips, nothing else was different between the 2.

so twin tips = stock pipe from something other than a GTVi which has been placed in the boot of a GTVi?

Is there a more precise way to identify what is a sports or VRX muffler from a stock one? i.e. a part number stamped on it

The Sports/GTVi/VRX is a single tip whether with chrome tip (VRX) or droopy (Sports/GTVi) does not really matter. These are what is called a straight through design with a slight s-bend inside. That is, offset in and centre out. The standard muffler is ironically called a turbo muffler which has baffles and redirectors which makes the exhaust go around a maze before it exits out of two smaller tips. Also, the AWD variants had the same mufflers, again with the VRX having a different tip. However, it is not a straight bolt-on to a FWD as the inlet pipe does not go over the axel, but instead comes in under the rear diff.

I found one in the boot of a GTVi stamped j20-06 with dual tips that point slightly down. Not sure if thats the original or someones swapped it

Edit: my KF Ei has dual tips

Part: J20-06 - This is just a stock baffled muffler and is the same part number muffler taken off my KJ AWD. It has a 2.5" pipe inlet with twin, short down facing outlets.

I'm pretty sure the VRX style mufflers were a "semi" straight through design in that there was a deflector or chamber in there somewhere? Offset/Centre configuration does not constitute semi straight through just because there's a slight angle to the perforated internal pipe. Technically, it would still be a proper straight through configuration.