I have a mould here for casting some slugs that is realy different. its made vrom brass and not complete, we think it was sold by Burgsmüller in Germany between the wars.

the bullets have two "windchannels" to make it spin by the air in flight.
we know today that this realy happen with such slugs but the speed of the spin is not quick enough to stabilise the bullet.

the previous owner had a special base to cast a spitzer bullet that looks like rocket science but I think it was a flat top originally

from the same source came another mould with the same design for the 9,1x40R, the biggest of the garden shotguns you see most times in 9mm Flobert rimfire