Some of the following is from various Web sites and some is personal experience:
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There were various experimental models of the PP, such as a ten-round model, and one with a decocker mounted on the frame. These never made it into regular production. Though the decocker can be found on the PP Super.
Here is a picture:
http://www.geocities.com/Pentagon/Quarters/2188/ppsup.jpg
The PP Super came about as the result of requests made by German police officials for a more powerful cartridge than the traditional 7.65mm cartridge that had been carried for so many years. The police were leery of the full-power 9mm Parabellum cartridge because of its penetration capabilities in densely populated Germany. Walther designed this completely new pistol in 1973 to answer their needs, however the cartridge was first experimented with during WWII but did not succeed.
This cartridge is the 9mm Police as Walther calls it. It has a faster, truncated cone bullet, better for penetration. It is similar to the Makorov 9x18, but the bullet diameter is smaller than the Mak; it is the same as the 9mm Para. This pistol was also chambered in 9mm kurz, which we know as .380.
It is a traditional blowback design with some added features that the Walther PP and PPK are lacking. First is a long-awaited for slide hold open catch. The second is a decocker instead of the safety lever found on the other PP series pistols. As can be seen in the image, the slide has been redesigned for a more modern appearance and the trigger gaurd and grip has been enlarged. It is very comfortable to hold and fits nicely in the hand. It is a larger pistol than the PP, approaching the size of the P5 in all dimentions except thickness through the slide. This pistol also incorporates the recessed firing pin first introduced on the P4 and subsequently used on the P5 and P88 designs.
Roughly 10,000 were produced in 9mm Police and about 1000 in 9mm kurz before production halted in 1981. Unfortunately it did not sell well in either caliber. It is nonetheless an excellent personal protection handgun and is built and finished as well as the other German PP series handguns.
At one time, about 15 years ago, perhaps, there were quite a few that came into the country and they were pretty inexpensive. However, as typical with all of the Walther PP series, and even with the truncated cone bullet, they were prone to failures to feed with a full magazine. Also the ammunition became a bit expensive. But it was more accurate than any of the other Walthers I'd had before.
Cartridge comparison data from Makarov.com:
9x17 = 9mm Kurz = 9mm Short = .380 ACP. Bullet diameter = .356"
9x18Ultra = 9mm Police. Bullet diameter = .356" (used in some Walther pistols...do not shoot this in your Makarov).
9x18M = 9mm Makarov. Bullet diameter = .364"
9x19 = 9mm = 9mm Parabellum = 9mm Luger. Bullet diameter = .356"
I next got a Walther P5 Kompac and got rid of most of my other Walthers. Later, I got my first Makarov and have not had a Walther PP series since. The P5 Kompac was however FINE in every way.