Not sure what you call the proper tool but a 20$ toolkit from alibaba does the job most of the time. Worked for my friends 15400 perfectly
My tool arrived just now. Going to order a few crystals from Prof to change on my divers and 44mm roos.I know this is an older thread, but if anyone is looking for the removal tool for the inner nuts you can get one here:
https://watch2parts.com/products/4-p...ch-parts-tools
I ordered and received this tool. It worked perfectly to take off the internal nuts holding the bezel screws on my 42mm ROO.