Starting in 1842, England has offered registration of it's decorative designs for pottery, china, wood, paper, pottery, china, porcelain, glass and more.
This protects both collectors and the companies who registered the marks.
In practice the codes can alter with numbers to reflect amended grants (C for example) but this covers the vast majority.
With each publication the earlier numbers and dates will be cited on the front page. Before then there was a single publication stage without As or Bs.
An item with a registry mark or number could have been produced before (less likely as the design would not be protected), or after the date of the registry mark.
The number listed for each year in the table is the first number issued that year. If your number is higher, but less than the number for the next year, then your item had it's design registered during that year.