What could this black sealent be? + Sharing my recent acquisiton and success!

I recently bought this watch for less than $15 - a beautiful small thing, I think it has a french case and a R11-5 movement, since it didn't work I was expecting a grimy/mossy movement, yet to my surprise the movement was spotless - partially due to the design, but I think the black sealent could've played a role as well

What could this black sealent be, it appeared initially as a welded case, but it's a very benign, easy to peel, yet impressively adhesive material



Loved this watch, loved working on it, and wanted to share this joy as well - ultimately the balance was too tight and it was probably why it wasn't working, took a play out of the russian playbook and managed to angle the bridge slightly upwards with a little piece of post it piece applied to the front left - so far it keeps time well!

It may be RTV, which would be quite inappropriate for a watch.

That's why it's an impressive material 😀

If I didn't scratch it off, it was keen on staying put, it was a very thin film too, so I'm guessing it was a thin fluid originally, otherwise it could indeed be an off the shelf sealant like RTV