@murrayjames,

当年dāngnián: a period of time in the past - it could those days or those years - not very specific.

当年，她还是个小姑娘。( You can tell from sentence like this, it's about things that happened in the past - not a specific year)

It also means the prime of life.

他正当年，身体好着呢.

当年dàngnián: the very same year. 事情发生的同一年 - It's more specific.

他们于当年3月份录制了《Kind of Blue》这张专辑。Because of 三月 here, so 当年 is the same year. It's talking about a specific incident here, so it can't be 三月 of a period of time in the past.

However, sometimes you do need context to tell the difference.

当年dàngnián needs a a little reference of the something else happened in the same year when it doesn't have a specific month or date.

Hope it helps~~;)