Anything is possible. This is Jaguar we are talking about. A car built this week is not the same as next week in our era. On the S Type Register we have proven this time & time again. There is no such thing as a typical S Type. This comes as no surprise as they were semi hand built.

My car has no attachment point on the scuttle or tunnel for an earth strap anywhere which is typical of S Types (of which I had 3 ~ 2 donors). All jump the LHS front engine mount including Cass' very late car.

I think the truth of the matter is that practice changed over time. From Mk2 method to S Type method. Mk1's I've seen jumping the engine mount were probably owners just copying later practice.

The modern Jaguar is a different matter. Using modern standards of build.