There is only one true design concept: Form follows function!
That's idiomatic in a purely mechanical world. It's always going to be true of a good spanner, and the inverse "form indicates function" is a natural consequence.
But that all rather falls flat on its face when faced with our favourite subject, test gear. The function of a frequency counter is a mile away from a signal generator, or scope, or SA, or measurement receiver, etc., but they all end up as rectilinear boxes of roughly the same order of magnitude and highly similar appearance. Indeed it's often necessary to understand the language that the scales and controls are labelled in to understand the function of such a device.
So it would appear that there
has to exist more than "
one true design concept".