Question: what is the quantity when the year of manufacture is 1900 and the class is MC?
SQL: SELECT "Quantity" FROM "2-18843924-6" WHERE "Year(s) of manufacture" = '1900' AND "Class" = 'MC';

Question: what is the class when the quantity is 4?
SQL: SELECT "Class" FROM "2-18843924-6" WHERE "Quantity" = 4.0;