You’re right, Jeff... it was manganese. It was used as an agent to make the glass clear, but when that clear glass was exposed to ultra-violet light, the glass gradually turned purple, or SCA (Sun Colored Amethyst). Bottle companies stopped using Manganese sometime after 1900, as it was a strategic element used in the manufacturing of stainless steel and other processes. I don’t remember what they replaced it with as a clearing agent.
Some unscrupulous bottle dealers would artificially ‘purple’ their clear glass to increase the value of their bottles. Not sure I got all the details of this 100%, but that is my understanding from my reading on the subject.
Jim