I'm currently designing new keys, which raises all sorts of issues that one mightn't at first consider.
In the old days, which for the purposes of this blog we'll take to be the days of the classic English flute, say from 1800 to1850 or 60, silver keys appear to have been fabricated, i.e. cut forged and filed from silver rod or bar. The cup would be made separately, from silver sheet, with the aid of a doming punch, which would raise the cup from a flat disc of sheet with one blow of a hammer. The two elements were then hard soldered together.
Later on in the period, it seems to have been more common for keys to have been cast, and later again when nickel or German silver became a common material for keys became more common again.
So one of the big issues for the modern maker is fabricate or cast, and the price of silver has a large influence on this. There's a lot of scrap silver, admittedly recyclable, left after fabricating a key.
The link above shows silver prices going back a long way, and looking at it reminds me that when I started making in the late 70s there was a huge peak in silver prices which in those days were almost the same as they are currently. So that was an encouragement to cast keys as opposed to fabricating them, and in fact to use other materials altogether such as brass (looks manky if you ask me) or ivory (beautiful but structurally poor, and with "moral" problems, of which more later).
The big problem ( there's always a problem, isn't there?) with cast silver is that it's really really soft. You might just about get away with short keys, but the long C and F are like sugar candy on a hot day, and will bend at the slightest provocation. The answer is work hardening, a process of hammering and filing which produces a key every bit as tough as a fabricated one, but at the expense of a lot of time, and the danger of making the shank of the key too small for the slot in the block.
A second problem is that the thin walls of the cups don't cast well unless the caster knows what he's doing, and they don't all (as opposed to the all don't which means something different).
I forgot to mention, of course, that before key cups as such, there were flat "flap" keys, but here the problem was finding a material that would form a good seal and still be thin enough. Leather was the only option back then, but it got hard and fell off, and was generally a bit of a bugger to attach and get to seal.
OK if you've only got one key, as with the baroque flute, but when you get to eight...it's no wonder that the key cup and purse pad soon took over.
If they'd had modern adhesives, and closed cell foam back then...maybe we'd still be using them.

They can certainly look well. Here's one on a flute that I was restoring lately. What's not to like about that, gorgeous engraving and all!
I'm being tempted to do a flute with flap keys.....and closed cell foam, of course.
Until next time...be careful out there.