INFORMATION:
This could be some form of battle damage. I do not think the battery was fought over. The American main thrust was directed to Cherbourg and the N/E area of the peninsular was left until last and most of the forces in this area were drawn back into the defence of Cherbourg The Americans did use captured bunkers to practice fighting in them to train fresh troops arriving in theater.

Above : a sign above the main door now removed?