Although Shadrach/Shadrack is a Biblical name from the Book of Daniel, it is not a particularly intuitive or common name. You may want to take a close look at the other contemporary Shadrach/Shadrack's of Maryland for additional clues as to possible family connections.

Also, consider the possibility that while the nominal bequest of 5 shillings may reflect that the son was already amply provided for, it could also reflect some disaffection, possibly due to religious differences. This WILLIAMS family may have been Catholic and disaffection created by the renunciation of the faith by the son, or vice versa.