Some citations in the OED have an italic a before the year to indicate the year is uncertain, for example:
a1556 N. Udall Ralph Roister Doister (?1566) iii. iv. sig. E.iijv, By gosse and for thy sake I defye hir in deede.
What does the a stand for in a1556? In this case, the book was believed to have been published in 1556, or shortly thereafter, but not known for sure.
I don't think it's "about" because it's italic so possibly Latin, and italic c is also used for circa in other citations ("c1315").