On the question of preservation of the XC 10, it's not a surprise as all are still in commercial service. Difficult to preserve something that hasn't been made available!
There were 12 latterly (two acquired from the LNER fleet), and I was thinking more of the coaches - none of which have been preserved, or appear likely to be.
The prototype Mk3 1xxxx vehicles received 4xxxx numbers upon reallocating amongst production HST sets. I travelled on two of the TS vehicles - 42000, 42002 - during the '80s.
Internally, the clue of their provenance was their bulkheads, midway along the passenger saloon:
Production Mk3 vehicles included a small window in the bulkhead panel.
Prototype Mk3 bulkheads, however, had no glazing pane.
They were renumbered in the 4xxxx range when in the prototype set (the power cars became 43000 and 43001). Subsequent renumberings of the five which went into production sets took place (some were again renumbered later) when they were converted into standard HST trailers. This was quite involved (there are photos of one stripped down in one of Colin Marsden's books), and included replacing the standard coupling gear / buffers with the HST arrangement of a fixed-head Alliance coupler and no buffers, removing the RCH jumpers from the ends and moving the 36-way control cable from beside to beneath the gangway. The slam doors were also replaced - the prototype set had doors of a different profile, and the bogies were replaced too.
The prototype was originally built with standard ETS, but one power car subsequently converted to 3-phase - I don't know which of the coaches were converted to 3-phase at that point.
The five prototype vehicles which were in TOC use aren't easy to tell apart from the later Mk3s externally unless you look closely - the main giveaways are that they don't have a raised window frame on the outside (which all the production Mk3s do have), and they have three small Roevac vents on the roof at each end (the production Mk3a TSO and FO also have this), rather than the single larger rectangular vent in the middle, which was fitted to the production HST trailers along with the Mk3a sleepers and the Mk3b FO / BFO.