what is the chrome metal button on the top of the shutter housing used for? Ive identified shutter cocking lever and the shutter realease and flash sync delay lever as well as the flash terminals. This is about a quarter inch in diameter button.

It's referred to as a Press focus button. Cock the shutter, press the button and simultaneously trip the shutter. The shutter get caught in mid exposure. Now you can ground glass focus, then cock the shutter to close it.

This way you don't have to change the shutter speed to open the lens.