A magnetic sensor assembly for rotary gas meters includes a counter module electronic volume corrector (“EVC”). In one aspect, the assembly includes a magnetic sensor probe configured to be releasably secured within an aperture of a first gas meter body. The sensor probe may alternatively be positioned within an adapter sleeve. The adapter sleeve is configured to be releasably secured within an aperture of a second gas meter body. In another aspect, the counter module EVC includes a base coupling member, and the assembly includes first and second magnetic sensor probes, the probe being configured to be releasably secured within apertures of first and second gas meter bodies, respectively. Each probe has a probe coupling member that is releasably engagable with the base coupling member.