Help me guess heat sink specs

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Is there a rule of thumb for estimating how well a given heat sink will work in free air. I need to make a rough guess for a pair of heat sinks I found in a pile of salvaged junk.

Each heatsink is a simple 2 x 2 inch "U" channel with small grooves and is 9.5 inches long. about 1/8th inch thick. What makes these nice is they have a socket for to-6 transistors. Here is a photo

https://www.dropbox.com/sh/ujepcbbgevxj75s/OMcwO7YYKT

Yes, I know I could measure then by bonding a dummy load resister to the heat sink and measuring temperature vs current in the resistor. But "what a lot of work" and I'd STILL need to make an initial guess.
 
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