The M/S slide switch with the positions "M" and "S" is missing in the models with the additional designation "A", e.g. "M517 A".
Supposedly the models M516 – M517 - M518 are technically identical! The three different model numbers indicate the different circuits and the different plugs that are necessary for the different Uher tape devices, so that the remote control of the pause button and music-language switching remain transferable.