Hopples are "gadgets" that are put on the horses legs to encourage/promote a long stride on cart horses (pacers / trotters). But they are usually put a lot higher (above the knee). I was thinking that maybe in training at a walk they might have put them lower, but I still think that's pushing it too. I think your girl just got cuts from something and scarred. Could be almost anything.
Here is a photo of my newest OT horse's damage -- this is the freeze firing (cyro) method, smaller marks than pin firing and in a lump, not a row:
This is pin firing on my other girl in a neat row on the back of her leg. The "clump" of white marks is probably pin-firing that someone tried to pass off as freeze-firing. Notice her severly bowed tendon. This is what they were trying to "fix".
Notice the difference especially in the size of the scar, but either way, it is very clearly artifical -- very round marks... Pin Firing is illegal on the track now, but I don't know about cyro. Regardless, people do it anyway -- off track and bring the horse back for the next season. Ya, try to prove who did it when...
Anyway, your horse's scars are not from firing. Be happy!