Barretters are resistors designed to pass a constant current independent (more or less) of the applied voltage. I.e the resistance rises sharply with increase in current. In the type illustrated this is achieved using a pure iron filament in an atmosphere of hydrogen. These were used to offset the temperature dependent resistance of valve heaters. When a series chain of heaters, powered directly from mains or HT voltage, is used, the overall resistance is very low when the heaters are cold. A barretter is inserted in series with them and effectively limits the current to a safe value while the heater filaments warm up and increase in resistance. The barretter may also protect circuitry if one of the heaters develops a short to ground. Often found in AC/DC radios.
The Sylvania 4A1 has a voltage drop of 4 V at 300 mA. It has a frosted envelope.