One of 8 died so I decided to see how heat sinking was achieved & if possible, why it died. It didn't give up it's
secrets easily but found that about 1/3 rd of outer shell is a Al. casting with a inner flat mounting surface.. LEDs
light up but too much remaining potting to trace circuit.

 

That is called a Metal Core PCB
http://www.bergquistcompany.com/thermal_substrates/LEDs/pcb-comparison.html

 

Heat sinking is very difficult for 40 watts. On the order of a degree C per watt.

My guess is that the heat sink was not sufficient. I have seen few lamps that I think have big enough heat sinks...