From the 1990 book, the conversions were all very late build Mark 1s. The 7xxxx numbers are the numbers the coaches carried in the 309s.
Not sure of the exact numbers but I know they were the newest Mk1 TSOs in the 50xx series
Is there a reason why the newest Mark 1s found their way into these units? What had those particular vehicles been used for immediately prior to conversion?
Probably because the last batches of Mk1 stock had 100mph Commonwealth bogies from new.
Yes, 59831 which was converted to replace Class 309 set 616's griddle car after it suffered accident damage, 59831 being renumbered 69108 after conversion. Ironically 69108 then suffered accident damage itself and was withdrawn in April 1981.
The SKs and CKs were from some of the last batches as well. Commonwealth bogies possibly a factor as mentioned by someone else. Also the fact they were all Eastern Region vehicles.
It is an interesting contrast to the coaches used for the Southern Region conversions which were admittedly a few years earlier but used somewhat earlier build mark 1s. I guess the Southern Region never really had late build mark 1s to start with.
Also add in that they were of similar age to the as-built 309 vehicles.