The undercut stems help air flow some over "factory style" valves. The bump is present on original style valves because a "generic" forging is used and there is no concern about performance , just how quick can they spit them out so the price is cheap. I have known Alex ( the vendor that you note) for well over 25 years and have had "many" dealings with him during that time. That includes many stainless valve sets. He buys direct from the valve manufacturers ( mainly in Brazil) who also sell to others that mark them up substantially for "perceived value" ( read more profit).
I highly recommend him!
Randy
