This is undisputedly the rarest 6138 Chronograph and, apart from the near-mythical Cal. 5718 Chronograph that only Seiko has in its museum, I would say is The Rarest of all Seiko chronographs.
Introduced in 1971, the 23 Jewels version of Cal. 6138B was in production for only 1 year, and cased only inside the 6138-800x, a Japan domestic model. Two dials are available, a black iridescent charcoal and a rarer pearl white that is hard to capture in photos.
Movement: Seiko Automatic Cal. 6138B, 23 Jewel, 21600 bph, bi-compax chronograph (60s / 30min / 12hr), QuickSet Day/Date, auxiliary hand-winding
Case: Stainless steel (diameter: 40 mm w/o crown, thickness: 14 mm, lug-width: 19 mm), screw case back
Crystal: Mineral Crystal
Water Resistance: 30 m
Bracelet: Solid-link stainless steel, signed clasp
Manufactured: In Japan, 1971
In 1971, the price of the Chronograph 6138-8000 would have been just below that of the King Seiko, but had you chosen to walk into a Rolex AD instead and bought an SS DateJust, you would be left with some change