antecedents

so we all know that engineers are Scottish - Scotty is and then Startide Rising reinforced that and of course Chicken Run did the same thing, but I found this:

"...it took more than a degree and a Scottish accent to make an engineer."

in Heinlein's Door into Summer (p 162 in this edition) copyright 1956 and 1957.

So where did the trope of the Scottish Engineer come from?