I was watching a video from USA Kilts about the origin of the knob atop a metal cantle. Their explanation makes sense though I sense it's missing something of in inaccurate in some manner. I've seen many photos of Victorian Sporrans. Most of them have a simple flap at the back of the sporran for coins and bank notes. Attire was more formal back then. I assume you'd have your jacket on all day. There would be some pockets to keep your tobacco and matches in.

The evolution of the sporran following the second world war makes a lot of sense. There was more to carry.

Here's the link to the video
https://www.youtube.com/watch?v=kp5HQ7BBoQ0