Unstamped 7 3/4 X 11 Ligne friction jeweled bottom plate in envelope marked 558. Five friction jewels.
I've been unable to identify the particular grade of this plate yet. It would certainly be a work saver in the case of a cracked jewel if you recognize the grade and need one. Cheaper and easier than rejeweling.
Or, if you are building your own movement you could save a lot of milling work by starting with this plate.
