Here is my comments on Pyte's TUNGSRAM RED 220-230 V 15 W E 14:

The copper base does easily corroded because there is not enough alloy to fight the oxygen in the air that bonds the copper atom, hence poor alloy mixture on the base of the bulb.  Brass is usually used on the base of the bulb to ward off the corrosion of the air so it won't bind the fixture when it used outside weather.