Thankyou for your explanations, Dave and Ken. It reminds me of the British Midland Saab 340s, which was a mixed fleet of -As and -Bs. The former flew largely from East Midlands whereas the latter were almost always based at Leeds Bradford. The -B was, I think, fitted with more powerful engines and therefore better suited to LBA's shorter runway, even though they were only operating sectors of just a few hundred miles. The -B might have been a higher weight aircraft too.