Sandy Salisbury who was in both the Millenium and The Ballroom confirmed with me that Curt Boettcher used a chamberlin, not a tron, on all of the recordings he produced. There is also chamberlin on the "Save It For a Rainy Day" (1966) album by Jan & Dean, "Tanyet" by the Ceyleib People, and "Don't It Make You Wanna Go Home" (1969) by Joe South. The chamberlin string sound is nearly identical to that of the MK II mellotron. The earliest MK II tron recordings which were made in the United States were by the Tokens on their "Intercourse" album of 1968 (they confirmed via email they did order a Mark II from England late in 1967) followed by the Fuse album
with Rick Nielsen later of Cheap Trick playing mellotron. The very first M400 recordings in the United States were on Big Star's eponoymous first album, Ardent Studios in Memphis bought the first white M400 sold in the United States (verified from Terry Manning, engineer on both of Big Star's first two albums and the one who played tron on Big Star's "Give Me Another Chance.")
Mike Pinder supposedly donated one of his MK II's from the Moodies to the Beach Boys in the mid Seventies. I listened to all of their albums and there is no audio evidence it was ever used by them on record. However, Elton John did record a track off his "Blue Moves" album at the Beach Boys Brother Studios in 1977 which probably features this tron.
Finally, I did obtain a copy of the Graham Dalley Dozen lp "Sounds International" and the only keyboard instrument on that record is an organ. The liner notes on the back mentioned he did play "mellotrone" on a BBC Radio Music Show. Thus the first recordings featuring the MK II are most certainly those by Graham Bond in 1965 (although these recordings are flawed as the tron was not hooked up to an amplifier and you can only hear it way in the back ground). Both of Graham Bond's later albums recorded later in the United States in 1967 and 1968 do feature one tron sounding track each. Being that these were recorded in California, and that Graham Bond's mellotron was later used by Czar in England in 1969, it is a mystery whether the two USA Graham Bond albums feature either chamberlin or tron. The earliest act to use the MK II tron most effectively was Mandred Mann commencing with his "As Is" lp in late 1966. The recent compilation "Ascent of Mann" contains 23 tron tracks, including several which use the rarely heard jazz rhythms of the MK II.
Hope all this info helps.
Chris Haley