A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Murray Buckman
Date: 2021 Jan 7, 16:06 -0800
Look for "Jesse Ramsden (1735-1800): London's Leading Scientific Instrument Maker" by Anita McConnell, Aldershot Ashgate Publishing 2007. In the book she refers to the bridge sextant and, like you, appears to be describing the shape of the frame in that it forms a bridge across the top of a pillar.
The book can be found for purchase online new or secondhand, but is rather expensive.