Voltage ratings on incandescent bulbs have to be taken with some
skepticism.  They are usually intended to be "nominal" ratings.  

For instance, a 12 Volt bulb intended for auto use will spend most of
its life working at 13.8 or higher and the people that make the bulbs
know that and take it into account.

Likewise projectors.  The 21 volt bulb may be intended to run at 21
volts or 24 volts - unless you see the manufacturers curves regarding
ambient temperature, color temperature of the light, voltage and life
expectancy you're just "whistling in the dark."

Sometimes a lower voltage bulb is used at a higher voltage - it
shortens the life, but puts out more light and puts out higher color
temperature light - so color renditioning may be better.

Likewise, under load, your 24 volts will almost certainly be lower
than 24V.