I'm going to venture a guess that it's an asterisk-shaped ring (*), cast into the piston as a heat dam, or maybe for expansion control. Some cast pistons incorporate Invar struts in the wrist pin bosses to allow for cam grinding the piston for minimum skirt clearance. Invar has virtually zero heat expansion, so the egg-shaped piston gets round as it heats up to normal operating temp.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!