Cylinder block connecting bolts question

Hi,

I'm in the process of pulling apart the engine from my 1999 wrx.
The bolts holding the block parts together are being a big pain though!

First issue is, they're down in the water jacket so my normal sockets are too thick to reach down

Second issue is they're super tight! I got a 12mm 12 point hex socket onto the bolt head but it's needing more torque that I'm happy to give it without knowing any more info.

My question is:
What exact socket is this bolt head? 12 point hex? tripple square? 12 point torx? I want to be sure before I really try and undo them.

Here's the bolt:


Here's where it is down in the jacket (sorry it's a bit hard to see):


and here's the diagram from the manual of the bolts I'm talking about:


Thanks!
sorry if this is in the wrong section, this is my first time using this forum :)