Forum:(MGS:PW) How can you unlock specific "Extra Ops" missions?

I've been playing Peace Walker for some time now, beating the game (including Chapter 5: Outer Heaven) and having unlocked many "Extra Ops" missions and weapons but I still noticed that there were a few gaps in the Extra Ops missions, like EO #062 was missing from the list, and so on. It wasn't until I bought the HD collection that I started seeing Extra Ops missions that I've never seen before like the "Monster Hunter" missions and what not. I heard rumors that there was some content only available in the Japanese version (I own the North American version) and was wondering if some of these missions fell under these restrictions.

Are there set requirements to unlocking specific Extra Ops missions? Or do you progressively unlock more by completing Extra Ops missions, and if so, is it required to complete missions with an A Rank or higher to unlock other Extra Ops missions? I haven't seen an online walkthrough that explains any of this, so I was hoping to find some information here.

Thanks in advance!