I actually like to imagine that the scientists of the Enclave simply used the older models and took years if not decades to reverse engineer the technology behind the power armor, as most of the knowledge was lost after the bombs droped, and later they just improve it leading to the Enclave version that we saw in F2.
But like I said, there is no reason to believe that the T-51b Power Armor was the only power armor that existed. Particularly if it was a proven and reliable technology before the great war. As Willy Rogers said, WW2 is a perfect example for a lot of it. WW2 saw a lot of standarized equipment manufactured in the 10 thousands, like the Sherman, T34, Panzer IV etc. but it also saw some very rare and exceptional designs. Most on the side of the Germans, but still. The M36 tank hunter and M26 Pershing heavy tank have been both relatively rare tanks compared to the number of Shermans that have been poured to the battlefield. For all we know, the T-51b could be the Sherman of the Power Armor where other, eventually more sophisticated models, have been specialiced units with low production numbers. I mean it isn't rare that many different companies get the task to design a tank, and they come up with different designs. Most don't leave the prototype stage, but still. Before the Pershing or any other heavier vehicle was deployed on the european battlefield the Ordnance Corps of the United States Army started development of several different prototypes and paper projects on heavy tanks as early like 1940. Who knows how many models of Power Armor, maybe even just a handfull of prototypes could be present in the Fallout universe. As long it just doesn't pop up out of nowhere and has a reasonable explanation for it's existance I am willing to accept different models. Like I said, the US was in war with China for a couple of years in the Fallout time line.
