Titanium Quartz by MONDAINE, ca. 1989
Note the clever groove in the rim of the
case at 11 o'clock
To open the snap-on case back just left of the lower right lug. I had found this on several MONDAINE watches (Mirexal is just the retailer's branding). Please find a more detailed description how to open those watches on the movement page of the gold-plated Mirexal.