Question about very old GMT-Master

[cuco]

Hi, back in 1969, I bought a GMT-Master, the watch of my dreams. Of course, the paperwork is long gone by now and I would like to find out some info about this watch. The only document I have about this watch is the Service Instruction Booklet. The number 2109XXX is perforated in it, and I assume it's the watch number. Above it and also perforated is the number 1603, wich I have no idea as to what it is for, or about. When I search for the number 1603, I get a reference to a Rolex Datejust, not a GMT-Master.

I just sent this watch to be refurbished, so I don't remember if it has any markings on the case.

Can anybody in this group can provide any information that will help me, after about 40 years, fall in love again with my GMT-Master :-)

Thanks in advance for any help,

Cuco
San Juan, PR