The Absolute ENCODER index S1D for rotary gas meters RVG, IRM-1 and IRM-3 DUO consists of a mechanical index in which each individual digit roller is optoelectronically scanned and read out. The data is then continually transferred as a data word via a digital interface to a downstream device (e.g. volume corrector), whereby permanent data transfer security is guaranteed, compared with the conventional transfer of pulses.