How to remove the back of the calibre 83? There is no notch at all

Hi Siduch.

I would go to a watchmaker near by. They use a wooden or plastic tool to open the caseback without scratching it.

BR

Hi OP, have you looked all around the edge with a loupe?  There almost certainly will be a notch, could be quite subtle, but someone mignt not have replaced the back in the correct position, opposite the crown at 9.

My cal 83 WWW has quite a subtle notch and fits the mid case extremely well and tightly.