This is something that is of interest to me as well. Desioner is correct that usually it is the girl that is the deciding factor, but there are some series that really bring it out in an actress while others that seem to actively discourage it or just feature such shoddy camera work that you can't really catch it.
Yuka Osawa
Ayu Sakurai
Miku Abeno
Tsubomi
Rei Mizuna
This is a good list but I can add a few other prolific names (Women I've seen do it in more than one title):
Megumi Shino
Tsukushi Osawa
Shizuka Kanno
Mika Osawa
Arisa Nakano
Reiko Sawamura
Ayumu Sena to a lesser degree
There are some series that stand out as being very prolific with this type of thing where actresses who don't normally do open mouth swallows will do so:
最後は、のど射 (Deepthroat Morphism) from Moodyz
ものすごい舌上発射ごっくん (Tremendous Tongue on Fire) from Moodyz
100発の精子飲む (Drink 100 shots of sperm) from Idea Pocket - only really the later releases
There was a brief period of time where this was in almost every gokkun release. Most of the MVG MVSD titles from like 130-150 have it, there are a bunch of MIGD movies with it. Nowadays if a release isn't from one of those specific series above, if you don't see a mouth opener in the screenshots it is probably not happening. Even with the mouth openers, it isn't a given as the ones they are using now are so flexible that they are often clench teeth swallowing.
The stuff older than that golden age of open mouth swallowing has almost nothing. You basically have one scene in Mao Tachibana's Gokkun Bazooka appearance, a few instances from Mai Sakashita and almost nothing else. There are a couple in random Jelly movies from WAAP but the camera work is so bad that if you blink you will miss it. There are none in any of the other Gokkun Bazooka movies, MDLD-243, the Gokkun Club series, the Gokkun Land series or any of the early MVG stuff as far as I know.
Knight's Visual is actually terrible for this. I've only noted it in 3 installments - Both of Yuka Osawa's swallow movies and Rei Mizuna's second swallow movie. Considering like 80% of their releases are swallow themed that isn't spectacular. Asfur is also pretty rare. Plenty of shots with the cum at the back of the throat but most of the actresses will not pull the trigger without closing. There is a pretty decent repository of this stuff that I can link in a PM because I'm not sure about the rules for linking outside sites here.
I would keep an eye on Mai Usami and Tia also. Both of them have done open mouth swallows in S1 movies which is pretty unbelievable given the nature of that particular beast. Mai Usami has escaped the clutches of S1 but now seems to be working for SOD unfortunately. If a good studio gets a hold of her for a proper gokkun film it is likely to be excellent. Same goes for Tia.
