Non Standard Radiator Key

I have older radiators, which have a small square faced lug (Smaller than the usual square lug on modern radiators) at the back of the radiator. I've been told that this key looks more like a small spanner with a four sided hole in it.
Does anyone know where I can purchase a key for this type of radiator?
Many Thanks
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In an earlier contribution to this discussion,

Well, this *is* a D-I-Y group! Measure the size of the square lug, get a strip of metal, drill a hole whose diameter is the same as the width of the lug, and file it out square with a needle file. Voila!
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Cheers,
Set Square
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