The 524B was an evolution of the 524A, adding 52x-series plug-in units for additional capabilities. The main unit can count directly at a 10 MC rate. Three of the plug-ins are heterodyne frequency converters that allow measurements up to 500 MC.
The image below is of a military FR-38D/U, made under contract with HP by Northeastern Electronics. It is functionally identical to an HP 524B.
Shown below are six of the seven available plug-ins. From left-to-right, top-to-bottom: 526B Time Interval, 525C Frequency Converter (100-500 MC), 525A Frequency Converter (10-100 MC), 525B Frequency Converter (100-220 MC), 526A Video Amplifier, 526C Period Multiplier. Not shown is the 526D Phase Angle Measurement.