CREATE TABLE table_204_430 (id DECIMAL, "model" TEXT, "origin" TEXT, "type" TEXT, "version" TEXT, "in service" TEXT, "notes" TEXT)

which model comes after m939 truck ?
SELECT "model" FROM table_204_430 WHERE id = (SELECT id FROM table_204_430 WHERE "model" = 'm939 truck') + 1
SELECT "model" FROM "table_204_430" WHERE "id" = (SELECT "id" FROM "table_204_430" WHERE "model" = 'm939 truck') + 1