Let's get this out of the way - there's only two watch companies with such strong reputations for making tool watches that can make patina - evidence of a life lived with the purpose that a tool watch was designed for - attractive.
 
Namely, a Swiss company that starts with an "R" and...Seiko.
 
And this is one of those watches, a 1967 Seiko 6619-8280 MACV-SOG Arabic dial automatic sport diver, with vintage Waltham military compass, new-old stock USMIL strap, and MACV-SOG patch.
 
This is one of at least four Seiko sport diver references trusted by the U.S. Military Assistance Command Viet Nam – Studies and Observations Group (MACV-SOG) and issued to U.S. Special Forces during the conflict. 
 
During the era these were made, Arabic numeral dial designs were rare and quite the departure from the norm.  It’s a safe assessment the Arabic numerals were one of the aspects MACV-SOG favored, and USMIL forces assigned to the secretive group were either issued these watches or instructed to purchase them at the PX before a covert ops tour.
 
A highly classified multi-service USMIL Special Operations unit, MACV-SOG conducted covert unconventional warfare operations prior to and during the Viet Nam War.  The unit conducted cross-border operations - interdiction, tapping of enemy commo lines, identification of targets, and the rescue of downed USMIL pilots - from Viet Nam into neighboring Laos, Cambodia, and North Viet Nam, with the USG infamously denying its existence until declassification in 1997.  SOG operations levied a high toll on North Vietnamese logistic supply lines and bases in Laos and Cambodia following the early 1968 Viet Cong Tet Offensive. 
 
But to operate in a non-attributable manner in these countries – where the USMIL wasn’t supposed to be – required atypical equipment.
 
Established in 1963 and principally run by Conrad “Ben” Baker, the USMIL's Okinawa, Japan-based Counter Insurgency Support Office’s (CISO) main mission was to issue standard and untraceable (read, sterile in appearance to offer a fig leaf of plausible deniability when USMIL forces were operating somewhere they weren’t supposed to be in the countries in and around Viet Nam) gear, clothing, and weapons to Special Forces in Viet Nam, to include the USMIL’s MACV-SOG.  It would cease operation in 1973, following the U.S. withdrawal.
 
The story of CISO and Ben Baker, these Seiko watches, and the work of SOG operatives is told by Frank Greco in his book, “Running Recon: A Photo Journey with SOG Special Ops Along the Ho Chi Minh Trail.”  The Seiko 5's ordered by Ben Baker have become much sought after in vintage military and Seiko collecting communities.  Want more on the MACV-SOG use of Seiko?  Read these two Watches of Espionage articles, here and here.
 
Per Greco’s well-regarded tome and our additional in-depth research, SOG forces used four different Seiko references.  Some were CISO-issued beginning in circa 1968 and some preceded this time (so-called “Pre-CISO” watches) – beginning with the 6619 (first the 6619-8280 here, followed by the 6619-8060); then the 6119-8100, 7005-8030, and the non-Arabic dialed 6106-8100. 

This Seiko 6619 MACV-SOG automatic comes on a NOS USMIL nylon strap, and with nylon strap, springbar tool, SOG patch, vintage Waltham military compass, and rugged travel case.