The 200A was HP's first product, available in 1939. The other versions are very similar, varying primarily in the frequency range, and the 'C' and 'D' versions have a lower power output capability. All 4 versions were available by 1943. The picture below shows a 200B and a 200CR for comparison (the 'R' in 200CR means rack-mount). Note that the 200C has 4 ranges instead of 3. Note that the 200A and 200C should not be confused with the 200AB and 200CD, which are an updated design from 1952.