Designed by Herbert Meidel, the P9 used a miniaturized roller locking system like that of the signature mechanism of most HK firearms.
The miniature roller locking system featured an extractor that was nearly vertical position on the new gun. Removed from the slide, the roller locking bolt appears below.
The earliest prototype had the external hammer as shown above. By the time StK 003 was made, the external hammer was gone. 'StK' stands for 'Stabsstelle-Koch.' It is an abbreviation that meant the P9 series was designed under the wing at HK headed at the time by Theodor Koch.
Vital statistics for the P9 include a design year of 1965, with actual series production commencing in August, 1969. The manufacture of the P9 continued until March, 1978. Serial number series for the P9 ran from 090 025 to 090 510 with a total of 485 produced.
The 7.65mm x 21.5 version, produced in a quantity of only 24 in the P9 series, had a serial number range of 090 001 to 090 024. These were prototype models and did not see widespread production until debuting as the P9S in this caliber, in 1973. The model shown below is a P9S, second in production series that began with serial number 200001.
The 'S' as added to the P9 pistol stands for 'Spannabzug.' (Schponn-Obtsoog) Translated into English, Spannabzug means 'double action trigger.' This is the fundamental design difference between the P9 and the P9S.
Herbert Meidel, as with the P9, was the technical designer for the P9S series. Fundamentally amazing in recoil abatement, the roller locking system was working so well in the rifles, that inclusion into a pistol had to be inevitable.