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You might want to try Torx screwdriver. There is a good chance that lobes on
screwdriver would bite into hex pattern; a little tapping on screwdriver,
while knob is supported on hard surface, might help that bite. And do not
forget that a drop of Marvel Oil (or any other type of penetrating oil) into
screw hole would probably loosen screw enough to remove it; socking for at
least a day is recommended.

A second tack would be to try grooved Allen wrenches. I would not know where
to find them, never seen a whole set, only leftover from sets. Once that I
tried, seemed to work quite well.

Also, a metric wrench might make an oversize one, you have to convert
dimensions.

Before using any Allen wrenches, inspect tip of the wrench. Sometimes, those
tips get distorted and worn out, so you are operating an undersized wrench.
Tips can be restored by grinding off the worn part; do not forget, wrenches
are heat treated, so do not overheat them on the grinder.

Best of luck,

Miroslav Pokorni