Welcome to our group. I am on digest, so behind the pack in responses. I have also had login issues again.
I am a bit confused with your reference to Singer 27, but then back to the Sterling in the same/ next sentence. I am going to assume you referred to the Singer 27 because the Sterling is a shuttle machine.
Your description could be of hundreds of machines. The heart on the bobbin winder, the belt, the shuttle, decals, the shuttle slide plates, the treadle iron descriptions do not point to a specific machine. The Srerling name, as mentioned above is a "badged" name used by Davis, Standard and Goodrich.
I realize you are having trouble posting a picture, but I guess it does not matter, since our recommendation is that no domestic treadle is a good idea for your intended use.
If you want to sew leather you need an industrial machine. There are ones for light weight leather. Although not a treadle, I have my dad's Chandler which is for upholstery. He also sewed vinyl, plastic and the like. I have multiple industrial machines and my biggest challenge is to find the right diamond shape feed dogs that will move the leather, but not mar it. By the way, my Singer 29 (not yet in service) is a boot patcher, a needle fed machine and one of my two Singer 31-15s is a treadle. Although the 31-15 is consideted a Tailor's machine, I can see me sewing my light leather on it. In my case, I repurpose jeans and service uniforms, with new canvas and soft leather. All of my other Industrial Sewing machines are either clutch or servo motors.
As mention, most Industrial Sewing machines are for Single type use... lingerie, gloves, buttonholes, jeans, ... This might interested you
https://ismacs.net/singer_sewing_mac...ny/model-list/
Side note, my experience with buying machines is different than that mentioned. My experience with the costs of treadles in the US has ranged from $50 to $500.
Best of luck in your venture. We'd be interested in what you decide.