What Are the Ruler Type Markings in the Centaur Tank Turret For?

Probably to communicate to nearby vehicles target bearings. Similar markings were applied to the turrets of warships during the inter-war period.

They were used by shipboard fire control officers to direct the howitzer fire to land targets on the Normandy beaches. The Centaurs were on landing barges but could fire at land targets with sighting by the fire control officers using binoculars.

"They were used by shipboard fire control officers to direct the howitzer fire to land targets on the Normandy beaches. "

Even more than this. Other personnel were trained to call out targets to the Centaur and Sheman RMSG crews to facilitate the beach run firing and even afterwards.

HTH