This is a 20mm recessed #0 Horseman view lensboard (140 x 140mm) which allows for mounting wide-angle lenses 90mm and shorter in Copal/Compur #0. This lensboard fits all Horseman optical bench (rail) cameras(LE, LX, 450M etc).
Typically this lensboard would be used together with a wide-angle (bag) bellows for optimum convenience.