The 503BG is no longer available from the manufacturer so we get them from time to time but not guaranteed. A replacement we have for this mic is a CAD Audio 119S-19 Here is a link to the page for it https://www.4tecdirect.com/cad... .  Replace the mic element in your 503BG with a new Shure R172, Close-talk microphone delivers excellent voice fidelity for fixed station applications with high background noise levels. Features include a hum-bucking coil to reduce induced noise and a frequency response tailored for voice communications.

Designed for indoor communication systems, including transportation, restaurant, and industrial applications. Suitable for environments with high levels of background noise.

Note: For proper operation, mouth-to-microphone distance should be 2 inches (5 cm) or less.