Vintage Princeton Tectonics bottom timer with rubber boot and velcro wrist strap, 2” diameter.  The face of the timer has some scratches and the velcro wrist strap has been resewn.  

The timer is wound like a watch using the knob on the top, when you descend to a certain depth (around 6 ft. or so) the pressure depresses the black button on the back and the timer starts.  When the dive is over, the timer is reset by pulling the knob on the top out and rotating the needle back to zero.  I used this bottom timer during my self-reliant diver course as a redundant dive timer and it worked well.  I have tested the timer by winding it and depressing the black button on the back and it appears to be working fine.