The seemingly confused year in the date of the Pilkington slab at All Saints’ Theddlethorpe, Lincolnshire, can be explained by the change from the Julian Calendar to the Gregorian in 1582, not formally adopted in England until 1752.
The Gregorian Calendar new year started on 6 April; the Julian Calendar on 1 January. Everyone changed over to it, but for His Majesty’s Governement, which is why the finanical year still starts according to the Gregorian Calendar.
A lady of 45 years would have expected 5 January to be in the Gregorian Calendar year 6 April to 5 April 1728. But by that time the Julian Calendar had come in, by which time 5 January was the fifth day of 1729.