It was cheap... Rs 4/- per piece. And it definitely is some product in the market...
Vendors in India usually are bufoons who have the faintest idea of what they are selling... Just know that it is used for this... Cost me this and I should sell it for that and make this much of profit.
About the heat sink... If it is not a heat sink, it ought to be something else... It surely has some utility.... I would like to know what is this object...
Description : Black plastic case that covers a TO-3 package on it's upper surface...The surface that has no pins.
Who knows...maybe it is a heatsink. Stick one in an oven not used for food and see if it melts at 100C. Or maybe you could just try and see if it works as a heatsink. Ive heard of non-metals like mylar being used to act as an electrically-insulating thermally-conductive interface between two surfaces. Probably won't work as well as metal though...its worth a shot. Does it have fins and stuff like that on it? Or is just a flat surface that attaches to a TO-3?
Just a flat surface... And I do not think it makes any good contact with the steel surface of the transistor as well...
I used heat sink compound on it. But it was already tried using a wire to a CPU heat sink (I did not have a proper TO-3 heat sink). So it didn't show any much effect.
try a little experiment - put one end in contact with a heat source (low wattage light bulb, for example) and then see how long the other end takes to heat up. If it's reasonably quick, it may be heat a decent heat sink material.
Instinct tells me that it isn't even close - plastic generally has some heat insulating properties.
They are just covers for the transistor, it's to prevent accident shorting from the collector - you don't see them very often these days. So NOT heatsinks, as you can tell by looking at them.