Oi! Linguiſt or perhaps 18C perſon! Long eſs?

Having ſcanned and uploaded a contemporary Engliſh tranſlation of a little-known Voltaire work laſt night (you can proofread pages yourſelf!), we ’re having ſome of the typical converſations you might come to expect in ſuch a ſituation: “Is n’t it intereſting that the long ſ is used in all caſes except plurals and doublings (as in “miſsives”)?” and ſo forth. It always comes up. And then we have a quick viſit over into the territory of, “And the ‘y’ in ‘ye olde’ is actually not a ‘y’ but a thorn, or ‘þ’, ſo you ſay it ‘the old’…”, I ’m left with a nagging question:

Why two kinds of eſs character, to begin with? Was there a time when they were, perhaps, pronounced differently? Or did they change as a consequence of ſome modern ſhift, perhaps in typeſetting conventions?

My hypotheſis ariſes from the ghoſt of that perennial (modern) proofreader’s nemesis, non-Germanics’ abuſe of the ‘ß’ character, which far too many technically aſtute folk miſtake for a Greek beta ‘β’, and which as a reſult many proofers need to repair in manuſcripts over and over and over and over….

But in my mind is that little queſtion-aſker: “Why is n’t the German character called an eſs-eſs”?