Sideslip Indicator not to be confused with a balance indicator is a rare breed.
Sometimes seen on a test aircraft fitted with a vane on a boom to measure the slip, away from the influence on the arflow by the aircraft itself.
Any multi having engines outside of centre line will be sideslipping during straight flight when there is any imbalance of thrust.
Sideslipping means more drag would have been crucial for a Concord so it was probably used as an aid in precise directional trimming.
Can ex Concordians confirm?