Complex sentence: The wheels, which were on suspension arms, were unlocked and raised for firing; the Mk1/1 had detachable wheels and the barrel with tripod attached, was stowed on top for towing.
Split into simpler sentences: The wheels, which were on suspension arms, were unlocked and raised for firing, Mk1/1 had detachable wheels. The barrel with tripod attached was stowed on top for towing.