OM617 Mystery Bolt?

[CloudControl]

Hello all. Please be gentle as this is my first post. I have learned a lot from reading through the forums and am thankful for all the helpful advice that y'all have provided. Thanks!
OK, onto this bolt that has me mystified...
The bolt is located at the front top of engine block, centered between the oil dipstick and the valve cover, sitting low. There seems to be oil leaking from around the bolt head and today after tackling the oil cooler lines (modified to AN fittings) and updating the glow plug wiring (thanks for all the info on here!), I noticed air bubbling out around this bolt. Yikes.
Does anybody know what this bolt is?? I've been told it has something to to with injection timing? I have scoured the forums trying to find info and to no avail. The Haynes and Chilton manuals have no mention that I could find.

I have provided a photo for reference.

I am looking forward to hearing your replies.

Thanks much!