http://puzzlephotos.blogspot.com/

I actually have a #303... Saw it at a tool swap meet a while back, and
was playing with it. Asked the owner what it was. Said he hadn't a clue,
but it was mine for 50 cents.



Erik
It is called a right angle prism and is used to find the point on a
line from which a perpendicular line may be run to another point, then
the true distance from the line to the point may be measured. this
information can be very important particularly in property surveys.
Gerry :-)}
London, Canada


I used one of these prisms for many years when staking out the corners of
test plots in farmer's fields. It's kind of hard otherwise to get the
corners square on hilly ground or when the crop rows are curved, like in
contour planting. Nothing near the accuracy of a transit, but fine for a few
inches in 100 feet. It sure speeded things up, especially when working
alone.
Carl Joplin