The bottom picture does not have a o-ring on it but I am sure that when you use it it does. There is a product called tef-gel that I highly recommend for these type of issues when you have dissimilar metals , salt water and electrolysis.
https://www.jamestowndistributors.com/userportal/show_product.do?pid=546&engine=adwords&keyword=tef_gel&gclid=CjwKEAjw5M3GBRCTvpK4osqj4X4SJAABRJNC7zP9DePnOKtDwCDlogDdySayZkzIuikH48oImcBZqRoCRDPw_wcB
Ultra Tef-Gel is a fastener lubricant used to prevent corrosion and seizing. Developed using aerospace industry experience, properly applied Tef Gel eliminates the blistering, corrosion, rust, galling, and seizing of hardware commonly experienced in corrosive environments and with dissimilar metals.
It can be used in all weather conditions. The amazing composition is derived from a specially formulated PTFE paste. Syringe tube design for easy application. Whether it swivels, slips or slides, Tef-Gel will protect and ensure the painless future removal of assembled hardware.
The nikonus connector is known for these problems. The o-ring should have been above the threads not after the threads.