What is this?

Hi all. So my watch repair kit has finally arrived from Mr Bezos' house of fun so naturally my first thought was to crack some casebacks off. I removed my Vostoks caseback and was greeted by this. What is this? I didn't even know Vostok made Hand wind watches. Any place I can read up on their history? Thanks!

Side note, that movement looks really good compared to the state of the case