It is not without melancholy that I wander among my recollections of the world of letters in London when first, bashful but eager, I was introduced to it. It is long since I frequented it, and if the novels that describe its present singularities are accurate much in it is now changed. The venue is different. Chelsea and Bloomsbury have taken the place of Hampstead, Notting Hill Gate, and High Street, Kensington. Then it was a distinction to be under forty, but now to be more than twenty-five is absurd. I think in those days we were a little shy of our emotions, and the fear of ridicule tempered the more obvious forms of pretentiousness. 

1)  Then it was a distinction to be under forty - 这句中的distinction前为什么可加a？ 查牛津词典，似乎应该是：

(a)[U] the quality of being excellent or important 优秀；杰出；卓越

a writer of distinction 优秀作家

(b)[sing.] the quality of being sth that is special 特质；特点；不同凡响

She had the distinction of being the first woman to fly the Atlantic. 她不同凡响，是第一个飞越大西洋的女子。

所以，麻烦老师讲解一下这里的distinction的释义和用法。

2) 顺便请教老师，查词典有时会看到一个单词[sing.]（使用单数），那么这种使用单数的[sing.]可否加a? 还是只能加the或零冠词？ 

3) tempered the more obvious forms of pretentiousness - more 是修饰形容词的比较级，还是限定数量的呢？如何区分呢？