Marc_E
(Marc E)
6
If you’re using a host that’s designed for a single LED the reflector that came with it will be longer than the optic you’ll be using for the triple LEDs, so the spacer is to fill up the missing length.
If this pill is to go into your own build i’m sure you’ll be machining the head to fit and therefore won’t need the spacer 
Yes, the thermal paste goes under the LED board to fill up the minute air gaps between the 2 surfaces, thermal paste conducts heat better than air.