These particular examples are claimed to have been left some time between when the school was created in the second half of the 18th century, and 1891 when it w

These particular examples are claimed to have been left some time between when the school was created in the second half of the 18th century, and 1891 when it was taken over by 'St John’s Church Institute, which provided skills-training and pastimes for young men from the parish church.' The longer 350 years refers to previous 'almost identical' examples found elsewhere. (Yes 'nearly 350 years ago' is a misleading title.) The National Trust took over the property before the war, discovering and preserving these artefacts during renovation in the 80s. It seems perfectly plausible to me, they just needed to stay dry.