According to the wiki…
Advance dating indicator: If an A is present next to the date, the
ticket was bought before the date of travel. Standard travel tickets
can be bought up to one year in advance.
Source: APTIS Ticket Features
After changing trains I asked the guard on the subsequent train. It turns it means that the season ticket was bought in advance of the start period … and now I see that it was, of course 20th March is more than 7 days away: the guard on the last train has sold me the wrong ticket. (I don’t know why he would sell me an advance ticket when I was already on the train.)
Credit:stackoverflow.com‘