What is this? Gun Mount

[USS ALASKA]

Sirs, while watching the video I posted here - https://civilwartalk.com/threads/wi...kely-rifled-cannon.209241/page-2#post-2787559 - this caught my eye. Below is a screen shot from the vid. I can kinda figure out what the big metal tracks are for - what is the purpose of the smaller ones highlighted in red?

Thanks for the help!
USS ALASKA

 

[Dan Kohli]

Could those rails be for shifting trucks, or for the wheels of a front pivot carriage setup?