Grand Seiko from 1965.
When I acquired it, the rust on the edge of the caseback was quite stubborn. It was in a much worse state originally.
I soaked rust remover into the gaps using a fine-tipped brush, then struggled immensely with the caseback opener, feeling like my brain might explode about four times. It was a formidable opponent after a long time.
I'm glad I was able to open it safely without leaving any tool marks.