The AC VTVM (Vacuum Tube Volt Meter) AV-2 came on the market in 1952 and was produced until 1956. It was the successor of the AV-1 that came on the market in 1951. In 1957 the AV-2 was replaced by the AV-3. In 1961 the AV-3 was replaced by the IM-21. In 1968 the IM-21 was replaced by the IM-38. So far they were all Vacuum Tube designs and quite look alike, but in 1976 the IM-38 was replaced by the IM-5238 that was an entirely new modern solid state design. Most other Heathkits that got the prefix numbers 52 just got a new look and feel, but not in this case. The IM-5238 was produced until 1981.