With a 5/16″ / 8.4mm single square and a 1/4″ / 6.4mm bi-square on a slight angle, this looks to have been straightened when compared with the EnFo stamped version. I reserve the right to cancel orders if this is not followed.
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Vintage Melco BK/3 Crank Box Key + Made in England + Car Tool Kit +
Small end: appears to be around 5/16” square (7.9 mm).