It was true that he had just bought a lamb, he explained, but his lamb was black.

1 一个简单的区别强调句型和主语从句外置的办法就是，去掉 it is/was that, 看剩下的部分是否是一个完整的句子。如果句子结构完整，没有多余的成分，则通常为强调句型。

你的句子中去掉it was that 后，形容词true无成分可作，这就排除了强调句型。希望你以后不要再问此类问题，自己用这种方法判断。

2 上句中黑体部分为外置的主语从句（即真正主语从句）。but后为并列分句。这个句子是自由间接引语结构。即引语作为主句，而引述分句（he explained) 以插入语的形式出现, 用逗号和引语分隔。如果改为普通的间接引语，则为如下句子：

He explained (that) it was true that he had just bought a lamb, but that his lamb was black.

这个句子中，引述分句为主句，引语为宾语从句。