Collars as icons of submission seem to be a relatively new script. Studio Biederer occasionally showed collars, as here, and they appear in BDSM prose at least as early as 1930, but they are fairly rare in the early 20th century, and not symbolically important.

A femdom pony-play shoot from the Biederer Studio, probably 1930s. The mask is kind of impressive, huh? I would guess it’s made from a real horse’s head.
I like how pleased the domme looks. But I have all these hang-ups about pony play. First of all, I don’t like ponyboys/girls on all fours. In my imagination, horses are big and strong and fast. If you’re on all fours, you’re a cat or dog or a badger or whatever, but not a horse. By the same token, I don’t like ponyboys with people riding on them, because I just can’t visualize them being fast and strong enough that way.
I’m picky this way….

Charles and Jacques Biederer were Czech expatriates living in Paris in the early 20th century. They set up the first BDSM / fetish studio of any real importance, probably around 1910. For thirty years, they produced vanilla porn and lingerie ads, as well as overseeing the soft/hard media shift and the inclusion of fetish wear in their own kink work. Virtually every major kink trope is represented in their work, and in several cases (spreader bars, as above, or bondage opera gloves) the Biederers are the first known example. They invented the vocabulary of BDSM as we know it today.
And then the Nazis dragged them off to Auschwitz and killed them.