3926 - 3946 TSO's from lot 30086 (1954-55) were converted to EMU trailers 1966-67 along with all bar 16 of the TSO's in the range 3998 - 4045 of lot 30149 (1955-56). Three of that batch were converted in 1975. Also 14 from lot 30219 of 1957.
21 CK's from 1951 lot 30019 (the first ever batch of MK1 corridor firsts) out of the 33 built all got converted as well along with a few others so I'm guessing that they weren't in bad nick or were worth refurbishing. A lot of them subsequently lasted until the end of MK1 slam door stock as well.
Regards
Guy