[QUESTION] What is the default sorting between ES 1.7.x and 7.x?

I am working on migrating from ES 1.7 to 7.4. There is one issue I picked up that is related to sorting.

In ES 1.7, if returned documents share same _score, the results are seems to be ordered randomly. Whilst ES 7.4 would sort by _id.

Could any ES developer help confirm what is the rule that ES 1.7.x uses to sort in this case? Much thanks in advance.