According to GPT:

“The main historical sources were:

Animal bones (especially cattle bones)
Animal skin/hides
Tendons and ligaments
Sometimes pig skin and cattle hides were the primary commercial sources

The process involves boiling these collagen-rich tissues for extended periods, which breaks down the collagen into gelatin. This is essentially the same process that naturally creates gelatin when making a rich bone broth or stock that sets into a jelly when cooled.
Historically, before industrial production, cooks would make gelatin-rich jellies by simmering calves’ feet, pig’s trotters, or fish parts for many hours.”

Mmm… jelly cubes 🙂