I've been pulling spanners for over 30 years. Never heard of a 12/16 bolt. It will be imperial not metric and from memory it's 1/4 or 5/16.
This might help you in the future. 6 mm, 8mm, 10 mm, 11 mm, 13 mm, 14 mm
.................................................1/4.....5/16.....3/8....7/16......1/2......9/16
Metric spanners f$%k imperial bolt heads and nuts.
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