A way to identify Gitzo models from pictures - On 300 and smaller series the locking collar for the center post is a rubber covered one under the top of the tripod; you reach in between the legs to get to it. The 400 series has a cast metal collar with two "ears", located on top of the tripod. The second of the pictures you posted appears to be a 400, with the "eared" collar showing rather well. The first pic is a 300 or smaller - no ears on the collar (it looks like the collar is hidden behind the front leg). The 400 series uses a rubber collar to lock all the leg sections, just like the smaller series Gitzos. Finally, the 500 series uses the eared centerpost collar, like the 400 and in addition, the top leg lock collar has two ears rather than a rubber ring. If you look at enough pictures on eBay, you will start to see these ears on the different series tripods.
Some of the older Gitzos didn't have a number on the tripod, although I think most heads had a cast-in number. However, the heads are interchangable, so it is possible to find a 300 or 500 series with an R4 head, which an eBay seller is likely to list as a 4 series Gitzo.
I can't find my older Gitzo catalog, but as I recall, each series was available with 3 or 4 (and maybe even 5) leg sections, and long or short leg sections.
I have two Gitzos - a 500 series and a 300 series. I like the 300 as to weight, but I do not like the centerpost lock between the legs. I much prefer the ears of the 500; it is easier to reach, and locks down better. If I had to start over, I would probably choose a 400 series based on this. Both mine have the rapid non-geared center column. I can see the geared version would be nice for a heavy camera, but for 4X5 or 5X7, I think the non-geared is fine and slightly lighter weight.
I agree the multi-angle leg spacing is a very useful feature. Sometimes if you are in the field right next to a large rock or low wall, you can angle one leg up to set on top. And of course sometimes you need to angle all 3 legs out to get low to the ground.
Finally, I very much enjoy the fact you have continued to ask follow-up questions in this thread. Because Gitzo offered so many options and combinations, it is a confusing subject. The only way to learn is to keep questioning. I'm looking forward to further questions.