Surveyor's Sight and Surface Level. Use?

Hello,

I have had this piece for many years and do not know it's purpose. I don't know how it came into my possession. I estimate age at perhaps 40? years old. Something to do with surveying, I think.

The paper with it calls it a #5500 Sight and Surface Level, by David White of Menominee Falls WI. I've searched for a similar piece without result. Have also consulted Wild Heerbrugge, also nothing.

It looks like it is to be hung from something, and when opened, there are two what I'm calling prisms? that when you look through them, you see 90 degrees left or right from zero.

Anyone know what this is used for? Still in use? Value?

Any help would be appreciated. Thanks in advance.