Sylvania FF33 replacement flashbulb
Sylvania FF33 flashbulb was a specialised flashbulb which used magnesium in its manufacture and was used to produce light over a longer duration period. This flashbulb was the same size as a Sylvania #3 flashbulb. The critical difference between the two bulbs was the Sylvania #3 produced a huge amount of light over a very short duration. The Sylvania FF33 produced a similar overall amount of light output but with a longer duration and lower intensity.
Meggaflash make an equivalent flashbulb to the Sylvania FF33 which is called the Meggaflash PF330 flashbulb. These are used in special high speed photography applications such as vehicle crash testing and destructive tests. This flashbulb is still being manufactured by Meggaflash.