P1 = Walther P38
P2 = Sig P210-4
P3 = Astra 600
P4 = Short-barreled Walther P38
P5 = Walther P5
P6 = Sig P225
P7 = Heckler & Koch P7
P8 = Heckler & Koch USP
P9 = Heckler & Koch P9
P10 = Heckler & Koch USP Compact
Source:
http://thefiringline.com/forums/show....php?p=3092654
Got it from google. Wiki vouches for the P210-4 and the Astra 400 as the P2 and P3 respectively, but I have seen a P4 in person and it is in fact a P38 with a shorter barrel. It never occured to me that it's P designation was one of many, though I know of all the guns you mention. Just clicked in my head after you pointed it out.
The guy on there makes an interesting statement that the "P" in the designation stands for "Police" and not "Pistol" (or their German translations rather). I don't know which is true, it could be either, but all of those guns were issued to police forces, some of which were specifically made for police like the P7, but some were designed with military in mind like the P1 and P8.
Pretty interesting stuff if you ask me. Glad you started a topic on it so I could research it.