CREATE TABLE table_77594 ("Class" TEXT, "Wheel arrangement" TEXT, "LCDR number ( s ) " TEXT, "SECR numbers" TEXT, "Manufacturer" TEXT, "Year made" TEXT, "Quantity made" TEXT, "Quantity preserved" TEXT, "Year ( s )  withdrawn" TEXT)

Which SECR numbers have a class of b1?
SELECT "SECR numbers" FROM table_77594 WHERE "Class" = 'b1'
SELECT "SECR numbers" FROM "table_77594" WHERE "Class" = 'b1'