G-Shock Mr G Database MRG-1 Caliber 1556, July 1996

Note that you should REMOVE the protective cover
from the Titanium case BEFORE opening the watch. In my case it did not matter, as the rubber had reached its end of life after 11 years. If you look closely, you see that the rubber had been "crushed" near the lugs. Even I had applied a very low pressure with the Bergeon's 5700Z speed sledge (watch case holder).
If the protective rubber coating cracks when you remove it, then this is a normal sign of ageing and you will have to look for a replacement part.