That’s the nitrogen gas valve you’re looking at .
I’m up to speed with you on the literature :
The referenced photo/video equipment adaptable (feature) correlates to using a digital camera (not wired video out) mechanically affixed to the rear diopter ring (removal of the eye guard cup).
There are many difficulties with adapting devices behind these scopes eye pieces due to the size of the rear ring. This is why you’ll find less than a handful of user videos on YouTube not to mention that the color and detail is usually severely skewed from actual image quality due to lens adaptations , spacing, and resolution limitations of recorders.
I’m currently working on a project to record through a D-750 white phosphorus myself - results have been slow and hard to attain….