This is the so-called ”+” belt. For the ”-” belt see 506915. Note: The difference in belts between 65-78 and 78-83 FL; FX models is due to a minor difference in primary housing length. Available belts are therefore designated as a – (minus; slightly shorter / MCS 506915) or + (plus; slightly longer / MCS 970974) belt; although the number of teeth is identical.