Rare black dialed version (only 550 black ones produced, 600 blue, 950 silver, per Rössler), reference A7688, 1971-73. Water resistant to 300m.
Cool watch. The 2572pc inside is a replacement... should be 2562pc. Service crown. Should be on a lobster bracelet. Someone wanting to set it right with all these things should factor in maybe $600 for parts (those lobster bracelets are going at around $400 now). Then figure labor, including a dab of orange lume in the paddle sweep second hand as the finishing touch. I think the asking price would not be out of line for a perfect black Defy Gauss, with bracelet, so I would negotiate accordingly.