The lamp base for amber and red lenses has two separate bulb holders which have 1 single filament 5w bulb and one 5w/21w stop and tail bulb, and there is a horizontal partition between the two bulbs.
The lamp base for just red lenses has a single central bulb holder which has one double filament 5w/21w bulb.
Corrected by SilverBeast, thank you.