See that light-colored stick thing on the lower right hand corner of the desk? Dad and Ted would walk this instrument across the charts to find our position. (Once when they didn't have the right chart, our position was halfway up Mt. Fuji.)
I want to call the thing "calipers" or parallel bars but I know that can't be right. It's for making parallel lines.
I think the one in this picture is the one we still have. (See below.)
Here's Skipper using a similar one at the desk in his cabin. I assume they used several over the years. They are probably pretty cheap.