A cylindrical tool string assembly for downhole data transmission comprises a cylinder comprising a transmission element such as an inductive coupler installed within a groove in its top surface connected to a transmission line housed within its exterior wall. The assembly includes a split spring ring housed within a groove formed in a bore wall of a tool string component such as a drill pipe or a downhole tool within the bottom hole assembly. The spring ring may be compressed and released when positioned within the groove. The spring ring provides a platform on which cylinder may be mounted. The split provides a passageway for the transmission line to exit the cylinder. The cylinder includes a recess in its top surface housing an extractor to facilitate removal of the cylinder. An anti-rotation lock between the cylinder and bore of the tool string component may prevent movement of the cylinder.