Sadly the fact that the type 3 bridle has holes in it to accept the rein pins isn't unique. Both types 1 and 2 have had bridles with holes instead of pins. Those just don't accept the reins for types 1 or 2 but are used by them selves sometimes.
bridles type 1 by
Tim, on Flickr
bridles type 2 by
Tim, on Flickr
Still they are different enough in look that you will mostly be able to tell them apart. I recommend getting at least one of each some time to compare in person and then you will have no trouble seeing them in auctions. As mentioned there are also smaller or differently shaped bridles for the ponies, newest style colts, donkeys, camels and even a bear bridle.
Here is one final photo to show you a unique rein type for the type 3 horses. it is dagged fabric covered and looks great for Medieval era settings. Sadly only in weird colors so far. They really need to do it in browns, silver and gold as well.
dagged reins type 3 by
Tim, on Flickr
Hope this has been helpful.