The professional line's high-water mark. The 6159-7001 carried a 36,000 beat-per-hour hi-beat caliber, a 300 meter rating and a one-piece case, and it did so in 1968, when almost nobody else was combining any two of those things. It followed the single-year 6215-7000 and ran only briefly itself, which is why it sits at the top of most vintage Seiko want lists and stays there.
Short production plus professional use is the recipe for genuine rarity, and this reference has it. Survivors are few, honest survivors fewer, and the reference trades at the level where every component decision was made by someone with a financial interest. It is also the spiritual ancestor of every hi-beat diver Seiko has made since.
The grail file. At this level, provenance and expert eyes are not optional extras; they are the purchase. Buy the seller and the paper trail as much as the watch, and let a specialist verify before the wire goes out. Patience is cheaper than a lesson.
Production windows and specifications compiled from documented collector references and period catalogs, cross-checked across independent sources; manufacturer records for this era are approximate by Seiko's own account.