Everlight 5mm warm white LED with inbuilt bypass diodes

The interesting thing about them is that they have a pair of back-to-back 5 volt Zener diodes connected across the LED chip. This not only protects the LED chip if it is connected in reverse, but it also means that should the LED chip go open circuit, then only the LED that has failed will go out, any others in the series string will continue to run as the Zeners will bypass the dead LED chip.

Part number is 334-15/X2C1-1SUA, datasheet for the slightly brighter UW bin version is here.