NavList:

Greg
I rather thought it might be something like that, possibly for use in hydrography from a ships boat along with a lead-line.  Since my previous post, I've found a very good explanation here http://sextantbook.com/category/sounding-sextants/  .  It's very clever the way the horizon mirror (except the device is used horizontally, so I don't know if it's still called that) has its clear part across its centre with the images from the two index mirrors brought into coincidence above and below the central object.   After laying out all your pocket money for a similar device, you don't need to lay out any more for a station pointer.  An endless supply of tracing paper will do, albeit somewhat slower to use.  Dave