Vergnano's book doesn't give altitude for 870 CV power, but I think it's meant at sea level, as mentioned by Denniss.
Power outputs are all in metric unit of measure, CV=Cavalli Vapore, each equal to 75 kgm/s. Often british sources neglect conversion, which is 1CV=0.986 HP (1HP=550 lbft/s). Curiously, german PS, initially wasn't quite the same, being expressed in lbft/s too and different for various regions.
GB