the lender would have received had the stock not been loaned. =

the lender would have received if the stock had not been loaned. 

我举一个类似的句子：

The millions of calculations involved, had they been done by hand, would have lost all practical value by the time they were finished.

句子结构：把省略了 if 后变成倒装语序的条件句，插在了主句的主语和谓语之间。

等于：

Had they been done by hand, the millions of calculations involved would have lost all practical value by the time they were finished.

等于：

If they had been done by hand, the millions of calculations involved would have lost all practical value by the time they were finished.

【翻译】如果这些计算是手工完成的，那么当它们完成时，所涉及到的数百万个计算将失去所有的实用价值。（句子暗示：这些计算不是手工完成的，可能使用了计算机）