GMT - Master - Tiffany & Co.

[JSam480]

I'm brand new and think this site is great. I'm not neccessarily a watch enthusiast yet, but I find all the info here interesting nonetheless.

The reason for my post is I'm curious about a watch I received. I know it is real as it was my grandfather's. It is a minimum of 14 years old since the passed away in 1994. From the slight fade of the blue/red paint I would venture, however, that is is much older. I know it's real because my mother had it appraised many years ago and certified as authentic. It's steel and I have no reason to believe that every part of it is not 100% original. I understand you need a special tool to open it, so at this time I don't have access to whatever info might be under the case. I have also discovered it is powered by movement. It's face reads as follows:

Rolex
Oyster Perpetual
Tiffany & Co.
GMT-Master
Superlative Chronometer
Officially Certified

From what I understand the Tiffany Rolex's a quite rare. Is that true?
Also, I do not have the original documentation that came with the watch. Is there a way to obtain that or an equilvalent substitute? If so, what is the most reputable company to go with? And how much would this cost?

Finally, what are these worth? I'm not interested in selling it because I'm fond of the rarity aspect of the piece and it's a cool family heirloom. However, I do feel I should insure it and am curius what the value range might be.

Photos are attached.

Again, I'm not looking to sell so please don't ask. Thanks in advance for any help I might receive.