Complex sentence: These were identical to the "SR666" prototype except for the Centaurus 18 engine and four-bladed propeller; at least 20 of the 50 aircraft performed in the aircraft's intensive trials programme.
Split into simpler sentences: These were identical to the "SR666" prototype except for the Centaurus 18 engine and four-bladed propeller. At least 20 of the 50 aircraft performed in the aircraft's intensive trials programme.