Here we have a 1980s Avia 666 feet devil diver, which has a 38mm Stainless Steel case. It is a Mid-size model which uses a “Monnin” case, patent no. 503,305 from M.R.P. Her. It is identical to the case used in the Heuer 1000 series of the early 1980s. This watch is an Avia branded variation of the Heuer ref 980.013. During the later stages of Heuer and the transition to Tag Heuer, cases and other parts were sold to other brands. You can find other divers watches from the period which use Heuer cases and dial layouts. 
It has a matte black dial with the Tritium luminous hour markers. It has a date window at 3 O’clock which is quickset via the crown. The hands are a silver tone with the luminous in-fill, in the Mercedes style. The second hand is white painted with the tapered design and luminous arrow tip. The uni-directional count up bezel has a coin knurled edge with the black and silver insert. The crown to the right screws down. The case back screws down. It is signed with the reference 3977 W20. Inside is the ESA 536.121 Quartz movement, which has been serviced, as well as a new battery fitted. It is keeping reliable time.
 
Full condition report below. Please see the images as these compliment and aid the description provided below.
 
Dial: 
The dial is original and is in decent condition. It is a black tone, with white minute track. It has Tritium luminous hour markers which have aged. There are some losses of the painted finish towards the edges of the dial.
 
Hands: 
The hands are original, of Mercedes style with arrow second hand. They have the original aged lume.
 
Glass: 
The acrylic crystal is in good condition with some light marking associated with age.
 
Crown: 
The crown is original and screws down. 
 
Bezel:
The bezel is stainless steel and is secure, turning as it should. The matte black insert is good, without major marking. 
 
Case:
The case is Stainless Steel and is in good original condition. It has a brushed finish with the polished bevel on the side. It has some light marking.
 
Caseback:
The screw back is formed of steel and is in good condition with light surface scratching. The original engraving is good. It remains unpolished. 
 
Strap: 
The watch comes on a new NATO style nylon strap.
 
Movement: 
The movement is an ETA 536.121 Quartz movement. It is original and is working perfectly. It has had a new battery fitted.
 
Case Dimensions:
Case width: 37.5mm
Case width with crown: 41mm 
Case length: 37mm 
Case length lug to lug: 45mm
Case Depth: 12.5mm
Lug width: 18mm