You asked for it!
So with a ball, you would normally rotate the material.
If you didnt rotate and hammered on, you would get the excess material squidging out to the sides, called flash. you would then let cool and grind or file it off.
Taking that into account it doesnt matter what shape element you put into the spring swage, it doesnt need to be symetrical. The top element of the sw picks up the impression of the upper part of the master and the bottom part the lower of the master. Trick is when your using your sw in production, to pre forge the shape as close as you can to the original, hence less squidging out to the side and a more accurate replication.
Making of the swage..
I used 2 mild steel blocks 40mmx30x80mm (suffient size), made the spring from about a meter of 30mmx8 mild, welded the blocks to spring.
I forged the master in mild steel, quenched, cut out some bits to make it decorative and case hardened it. I clamped the reins of the sw together so the blocks remained closed tight in fire, (this reduces scale build up on the face of the blocks and creates a better impression (so does a clean mug of coffee to a customer). Took my sw to the fire, got an good even heat on both blocks, get your cold master and centrally place between the hot blocks of sw, forge under massey until both blocks touch or close and be careful not to over hammer.
Hope this threw some light out, let me know what you have in mind to create and PM me if you need a dig out!