1/4-20 I think. 1/4" diameter rod, 20 threads per inch. Dead standard thread size in the US. As Erin says, it's the same as a tripod.
Are you flush-mounting? An LMS-520? The Lowrance flush mount kit was a couple pieces of stainless bar, about an inch or so wide, formed into square "U"s. The first bolted to the mounting thumbrew holes, and had a couple of studs off the back. The second was longer (the legs of the U), and a touch wider, such that bolting the base of the U's together caused the legs to press against the back of the panel, pulling the unit tight against the panel front. Not really much for a $30 or so mounting kit.