CustomChevyGuy, the P220 has undergone a lot of changes over the years.
The one in my post above is from 1986. It has the stamped sheetmetal slide (which was state of the art aircraft technology at the time). The breechblock is held in with a doubled roll pin. The muzzle area is electron beam welded in place. The older grip you liked is probably like mine.
The P220 slides today are made in the USA and are machined from a solid steel billet. They are stronger but weigh a bit more. Currently the grip is a one piece ergonomic grip, which is a love/hate affair, apparently. I like both the early and the later grips, but some hate the later grips. Right now it is the only plastic grip option on the P220, but two piece wood grips, and there are Hogue G10 grips available. Hogue is making most if not all of the wood and G10 grips for SIG.
P220 Nitron Full-Size
This is an example of the latest version with the one piece ergonomic grip.
As a generality, you should be able to put a two piece grip on a model that has a one piece grip. However, my P227 is early enough it lacks the screw holes in the frame and requires adapters. I don't know it the P220 always had screws or if the early one piece grip models lacked the screw holes. Take the magazine out and see if you see two screw holes on the frame on either side of the mag well.
You can see all of the current factory grip styles for the P220 here:
P220 | Sig Sauer
SIG has some plastic grips available:
Black
Grip Set, Black Polymer, P220 SLIM FIT, 2-Piece | Parts
FDE
Grip Set, Flat Dark Earth, P220 (As Used On Combat) | Parts
The above will fit later model SIGs with the slim mainspring housing. They WILL NOT fit an older SIG like mine with the larger mainspring housing.
Hogue has a plethora of grips for the P220:
Hogue Inc.
Edit: You mentioned a "standard rail" on one of the SIGs. You mean a slotted rail or was it just a smooth frame? The reason I ask is the first SIG rails we slotted but unlike a Picatinny the profile had a slight curve to it. This caused complaints and SIG now makes the slotted rail with a flat profile, more like a true Picatinny.