They are the dual-filament type....21W and 5W bulb.
The 21 is turn indicator and the 5 is running light.
Are you getting a fast blink on the turn indicator or just the bulb out with headlights on? Sounds like the put in a single filament bulb.
The 21w are hard to find a auto stores - as they don't usually tell you the wattage of the bulbs - just that it is 12 volts......