Ah, yes I can see the other button holes now you mention it. For all we know the women in that photo might be wearing men's breeches after all - it was common enough to re-use the wardrobe when the men were away.
However, according to what Lei wrote, these are only breeches for other outdoor sporting/manual use because jodhpurs, like the example you showed, have reinforcement around the knees/inside legs.
You know I have a couple of vintage breeches I've never troubled to look into, I shall go dig them out.