The drive keys I see are cast in to the lead slug.
They do not appear to protrude enough to contact the rifle's bore or groove
My sense is that they are there to positively transmit the rifling's drive to the projectile.
Looks neat.

I'm with Daryl on filling the bore - might as well get the most out of those nice 20bore tubes!

How has your further testing of roundball been working out?
Have you tried any more hardened roundball and different powder charges?
The hard roundball is sure to not tumble through your game animal...

Cheers
Tinker