Today I picked this up from Craigslist, Mitutoyo Height Master 515-310 with matching 12" riser block.  With proper procedure and operation it will read to 0.000010".    Now I guess I need a 50 millionths indicator.

Nice!  Good start for a tool porn thread

I'm looking at that thing and probably feeling like my wife does when I show her my awesome CL tool purchases. No idea what it is but it's beautiful.

It's a height gage.  

The steps are actually a gage block stack that moves up and down via the micrometer dial on top.  It's main gradation is 0.001", the sub reading is 0.0001", and the vernier will read to 0.000010".  We've gone from splitting hairs to measuring in helium lightbands.

Say you wanna know the height of a block, or dowel pin or some other datum. So you put it on your surface plate and sweep a test indicator across the top of it with a surface gage.  You then swing the test indicator over to the height gage and adjust the gage with the dial on top.  When indicator zeros you have found the height of your feature.  You could of course do this same thing with a stack of jo blocks, but this has panache.