"Primus" Magnum Finder

i really like finding 'unknown objects'.
this is my latest find in a Morningside charity shop.
I have no idea what it is for, the person in the shop had no idea either.
just what i like!
i also have a liking for lenses of any sort, i have some vague idea of using them for some art project of the future - i just havent discovered what it is yet.
Usually when i find a lens it is a magnifying one, this is an un-magnifying one - not sure exactly what you call that.

If anyone has any idea as to what it is for i would be interested to hear!
i think it must have to do with horizons because of the red lines drawn across - as if it is to be held up to look at a landscape or townscape .. uummm no, maybe not.. that idea falls down here, i cant think what it would be for after finding the horizon. if it was joined on to something then it might make sense...
i think i give up!