Please help identify my vintage (??) Longines.

I have just inherited a vintage (I think late 60's) Longines mechanical watch from my father. Despite all my attempt identify this watch from Web and other reference materials, I wasn't able to figure out model #/movement type of this watch. I also trying open the case to inspect the movement, but I wasn't able to so. Could anyone tell me more about this Longines? Do I need a special tool open it? It also has two numbers on the back of the watch: "16" and "835534".
Thanks in advance for your help.

The numbers on the back are the case serial number, and it tells me it's a 70's watch. Around 1969 Longines restarted their movement serial numbers at 50,000,000 but kept the case serial numbers going continuously. The restart happened somewhere around 15,000,000.