Acorn wanted to break into the US market, and so they had to redesign the computer with a massive metal box inside the outer plastic case.

Their attempt to launch in the US was a huge failure, and most of those computers were shipped back to the UK and "unconverted" to be resold in their home market.

But they didn't remove the metal box. So Brits could always tell when they had an ex-US BBC Micro because it weighed twice as much and had a huge metal box inside it.