With a previous number of 68165, what is the oldest converted date?
CREATE TABLE "great_northern_section" (
  "number" text,
  "previous_number_s" real,
  "previous_class" text,
  "converted" real,
  "withdrawn" real,
  "disposal" text
);
SELECT MAX("converted") FROM "great_northern_section" WHERE "previous_number_s"=68165;