Really not much difference at all, only the name (except that you can get a steel-framed P38, but not a steel-framed P1).
Later P1's do incorporate the steel hexagonal reinforcing pin found on all P4's and P5's, and that's something that is useful to have if you're not looking for collector value, since it reinforces the alloy frame at a known weak-spot in the design. Of course, steel framed P38's don't really need this feature.
I have this to add. P38 is the Nazi german designation for that pistol. P1 is the Bundeswehr or post ww2 designation for said pistol with a new alloy frame. Walther confuses us by naming the comercial version of the pistol P38. Also, post war steel frame P38s are comercial only and very rare.
One of the differences between the P1 and P38 is the barrel. P38s have a one piece barrel. However most P1s have a two piece barrel. The P1 barrel has a 9mm barrel liner inserted into a sleave. You can see the end of the sleve protruding very slightly from the muzzle and there is a seam on the top of the barrel near the breech. I don't think anyone really knows why Walther used different barrels in these pistols.
P38 police guns are the same as comercial guns and will show Ulm proof marks. P1s are military and have the eagle Buro mark instead of the proof.
To make things a little more confusing some P38s went to the military too and are the same as comercial guns however they will have the military Buro mark and no proof. They will also have the comercial Walther slide markings.
Be careful buying a P1. Many of them look like new but are assembled from parts. If you know the Walther design changes over the years you can spot one that was assembled by an armorer from parts.
Newscam, great insight into the barrels. I wasn't at all aware of that.
As for P1's being made from parts, I'm sure that's happened, although many of the P1's I've seen for sale of late seem to have the same serial number on the frame, slide, and barrel, so that seems to be a pretty safe bet that it's been a whole pistol since its inception and not a grab bag of parts.
Be carefull of matching numbers. A great many of Walthers pistols come out of the factory without a number on the slide. If you look at the slide numbers on a Bundeswehr P1 you will almost always have a number that matches the frame. But look closely at it and you may notice that the numbers are stamped with a different style dye or the may be not lined ou perfectly or may appear to have the number stamped by hand.
As collectors of German guns we often are misled into thinking that if the numbers match then the gun left the factory with all these parts. However the numbering system is not intended for us collectors. It is ther so armorers don't mix up parts from one pistol to another, when they are being serviced. At some point in their service life the armorers will always stamp the serial on the slide if it is not already there. If a pistol gets a new slide the armerors stamp the serial number on it. Sometimes they will reuse a slide from a broken P1 on another frame. They will then cross out the number and stamp a new one on it.
The thing to look for is a post 68 fat slide on an old frame. If a frame has ho hexoginal steel reinforcing bolt on it, it mean it was made before 74. I have seen many with a fat slide on the old frame with date of like 79 or 80 on them that look good. But if you see this combination of parts, it is definately a parts gun.
Most certainly does help Rich. I double-checked my 78 date coded P1, and not only were there no overstamps, but the last three numbers of the serial number on the frame were the ones stamped on the barrel and slide, so it appears authentic.
"P1" was the Bundeswehr designation for ALL the ones that they ordered from 1957 on; Walther did not put the "P1" (as opposed to the "P38") designation on Bundeswehr-designated pistols until 1963; from 1963 on concurrently produced military contract pistols were stamped "P1" and commercial pistols were marked "P38" or "P38II" (briefly, when the "fat slide" debuted in 1968-after a short time Walther reverted to just the "P38" designation. And excellent information source is Dieter Marschall's book "The Walther Pistols-Models 1-P99" available from Earl's Repair (www.carlwalther.com). There were also some differentiations made in the acceptance markings.