It is a slip indicator, but actually really the primary rather than secondary....
The reasoning is that the indicators attached to the EADIs, if you look closely, are adjustable via a friction screw. If the EADI is changed the aircraft strictly needs to be levelled and the slip indicator adjusted.......like that's gonna happen.
The one under the MCP is not subject to this issue.
A bit of a distant scan, I agree!
Health warning: I haven't seen the inside of a 767 for eight years......