So who developed the T-60.

Until this game we had never seen the T-60.

The T-51b was the apex of power armor technology prior to the end of the world...now before you say, yes I know in Fallout 4 there are two soldiers wearing T-60 armor guarding the entrance to vault 111. I know Bethesda now has rights to the canon and such, however I see this as more of a good than actual take to the bank canon since prior to this one scene the T-45d and T-51b were the main armors with the T-45d being far more common.

So if we ignore that one thing, who would have created the T-60? Is it a brotherhood creation? Or a prototype that never saw use before the bombs fell?

On the terminal at Fort Strong, It talks about improving the T-51 power armor. Im assuming this is what lead to T-60. Fort Strong was changed into a research facility. The "improved" T-51 was also deployed in Alaska. The terminal also hints at the T-51's weight was an issue and after a large fall, the power armor tends to tip over, Since we see the T-60 at Vault 111, the BOS have it with Vertibirds...maybe the T-60 was to be used for air mobile units, like air cavalry. What bothers me is advanced power armor found in different locations in the game. It is plausible a group of Enclave remnants made their way to Boston. Hell anything is possible. Deacon has been to DC

Are there actually sets of X-01 strewn throughout the game? All the X-01 pieces I've found have been part of levelled loot lists (the abandoned house, etc).

I assume it was designed by the CIT as a military contract then constructed in the Commonwealth and readied for for homeland deployment but never got beyond so.

I'm taking it as a retrofit of T-45 suits to incorporate new technologies found on the T-51.

See what makes me wonder is if a upgraded power armor, that is better than the T-51 was being worked on why did the Enclave have to sacrifice so many to develop the Advanced Power Armor? It took them many tries to even begin finding a better armor over the T-51 and a whole lot of people died doing it.

So much so that the classified information on the APA due to morale. If the T-60 was in development why not just use it as it is superior to both the T-45d and T-51b.

The Enclave built advanced power armor as part of there plan to retake the mainland. In FO2 it had a radiation resistance of 60. The Enclave seemed to be very concerned about radiation and it's effects.

Of corse they believe they and the vault dwellers are the only pyre humanity left and that radiation damage made wastelanders sub-human mutants, weird though in new vegas the remnant power armor had lower radiation protection than the T-51b suit, I'll just chalk it up to lack of maintenance.