From what I can tell about the part number....C5 - 1965
M - Mercury Car line
Y - Lincoln/Mercury Parts Division
6582 - Valve cover basic part identifier number
H - would probably have to do with the engine series and design of the valve cover.
Is it worth anything? Don't know sincee I don't really know what it looks like. But I am sure someone some where is looking for one like that. 
Post a pic so we can see what it is.
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Larry
No fast Fords at this time but one fine cruising 96 F350 CC DRW Power Strokin diesel.
And one rusty 64 Fairlane nick-named the Rust Bucket....And sometimes called the Money Pit...
The sounds of a brain dropping into gear.....
"Buzzzzzz.......Click"