u/OppositeSquare

I wanted to change the battery on my Omega Constellation. I found a reference guide for an ETA 255.561 movement because I was a little currious to see if everything matched the diagram.

I opened the caseback and noticed this black piece of plastic that covers more than half of the inside of the watch. I can't move this plastic because it's screwed in place. Besides the plastic, the diagram is clearly a little different from the watch. For example, there is a cog that is visible on the watch, but not the diagram - I circled this in red in the last image.

Is there something I'm missing?

I found the manual on watchGuy.co.uk: https://watchguy.co.uk/tmp/ETA%20%20255.511,%20255.561.pdf