I was thinking of ones that were more than a valve relief on the piston top, it was more a pair of scallops cutouts. Similar to the Rob Beere Ultra pistons.

 

The above is an SA favourite. Have no idea where they are sourced from. I must ask. The virtually flat top wedge is apparently the secret to swirl behaviour. A pretty simple design. Comparisons would be interesting vs. the more exotic.

 

The asymmetric design goes back to the sixties or earlier. Klat and Watson made some for the Lumsden E type. Repco also produced some for Bob Jane's Mk2 around the same time. I wonder who was first? To me, it's a very natural way of compensating for the asymmetric position of the spark plug. If the plug can't go to the centre of the chamber, then bring the centre of the chamber to the plug? I don't know of Jaguar themselves ever trying the idea, though they did some quite complicated machining, non-hemi or double hemi, of the combustion chamber to improve combustion in the lightweight E type engines.
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A few phone calls & Repco, Arias, & Wossner are the suppliers of these to SA. Apparently made in small batches from time to time. (forged)

 

Some of those pistons have a crown thick enough for machining down to adjust the compression ratio. It might allow a good shop to introduce a little asymmetry or at least a bit more space around the spark plug. Even without that I'd hope the 'cone' would stir things up more than the standard 'dome.'

One of the adverts (Wossner?) claims special treatment of the crown and then says you can machine it to your target c.r. No idea where that leaves the special treatment?

 

Me neither Peter. I guess it depends on thickness like an Alusil bore.

 

https://www.shopbhp.com/products/jag...ed-pistons-kit

'machinable crown' ... 'crown polished to better resist detonation'

So I use some Brasso to put the shine back!

 

Exactly! Not that in use it's going to stay polished for long. Ourselves & BP's additised fuel are best for combustion chamber deposit control in latest & ongoing testing. We remain the gold standard for valve tulip by some margin with Techron.