The Combine Soldier is a transhuman enemy fought in Half-Life 2 and its episodes. It went through a long and diverse revision history, a lot of which pictures have been released for.
There is a large amount of art of the Combine Soldier, radically different from any versions we see in-game.
Early Suit Designs
A few early concepts show different ideas for the Combine Soldier's design. For example, the "flames" soldiers reuse the head from some metrocop concept art.
Early Color Schemes
These are from a later point, when the general design of the Combine Soldier was decided on. Some of the concepts here experiment with a different head, along with coloration and smaller body details.
It's worth noting that in Half-Life 2 Survivor, the Combine medic models make some use of the alternate head designs.
Versions 1-3's order is speculation. If proof comes around, change 'em!
Version 1: Pre-2002
This is the earliest version of the Combine Soldier that there are screenshots of. What makes this one distinct from the others is the visibly red eyes without fingerless gloves. It has the red arm-patch in common with Versions 2-3, and helmet in common with Version 2.
Version 2: E3_Hydra
A strange version that is apparently a mishmash of Combine Soldier and early Counter-Strike assets. While the helmet is white, it seems to be more detailed than Version 1's. This is the only shot it appears in.
Version 3: 2002
The earliest version of the Combine Soldier we have the texture for. Unlike the earlier versions, this one has a navy blue helmet, and yellow eyes.
Soldier_Stripped from the Leak appears to be from this version. The pants textures from this version are identical, and the boot textures while moderately different, are still fairly similar. Oddly, the hand textures here are recycled directly from the Gasmask Citizen.
Version 4: Leak
The first (and last) early combine soldier we have the full model of. The helmet color has been changed to black, the emblem has changed to a blue-and-brown 314, and the eyes are now a darker, reddish brown.
Version 5: Early 2004
Far further along, this version of the Combine Soldier is nearly the final, but with some differences. The armor plates are the same color as the rest of the suit, and when comparing this to the final version, it has noticeably less shading.
- Stangely, it is used instead of the final texture for the GMod player version of the Combine Soldier and the Zombine.
Version 6: Retail
This is the finalized version of the Combine Soldier in Half-Life 2, varying primarily in shading and plate color from Version 5. No official screenshots are known to exist of this, as the Combine Soldier changed to Version 6 fairly late in development.
Version 7: Episode 2 Retail
This is final known version of the Combine Soldier in the Half-Life series. The the only difference between version 6 and version 7 is that version 7 has a dirt overlay in the texture sheet, to fit the outland theme of Episode 2. Apart from that, much is the same, even the shading is the same.
Texture Sheet Comparison
We have 5 revisions in total.