Isthmus touched on a very important point, case back corrosion pitting is a killer. I have a very nice Seiko 6309 that has corrosion pitting in the caseback and case gasket groove. They are too deep for the rubber to compress down into and prevent an otherwise nice looking watch from reaching its proper potential.
If you want a vintage watch to be WR the case back and gasket groove in the back should be free of pitting. Unfortunately, the type of stainless steel Seiko used in its older watches does not resist salt corrosion as well as the newer steel used, a pity because many were purchased solely for ocean diving. (Rinse, rinse, rinse!)
Just a few that have been returned to acceptable levels of Water Resistance:
[You may note that this one may not be all original.]