The star-shaped (12 point) post you showed is a “Type A” gas post. As others have said, you use a 7/8 boxed end wrench for this. You didn’t show what the liquid post looks like. It should be a six-sided post and take a 7/8” wrench as well. If the liquid post takes and 11/16” wrench (Type B), the posts are mismatched, and one of them is wrong for this keg.
Type A posts came on kegs stamped with one of the following manufacturers mark: Spartanburg, Firestone V Challenger, Firestone VI Challenger, Firestone Super Challenger.
Type B posts came on kegs stamped with Cornelius.
There is a Type C post but they came on older kegs which aren’t seen too often.
The Type A kegs have different post thread sizes than Type B kegs. They are close enough that you can sometimes use the wrong posts for a keg, but the threads get damaged. Sometimes you can’t get them to hold gas. The different post types use different poppets, too. If you use the universal poppets, that really doesn’t matter.
I just bought some used kegs online, too. They had mismatched posts, and one didn’t seal. I bought the right posts and universal poppets and all work as they are supposed to work.
My recommendation is to use what you have as is. If a problem arises, change to the right posts. If your Type A post is on the wrong type of keg, change it right away. Those low 12-point Type A gas posts are a pain in the butt to deal with.