yz125 piston help!

[elephant35e]

i was replacing a piston and my dirt bike and i noticed something different about the new piston.

they are the same size, but the new piston has 2 hollow cavities (or whatever they're called) and a rectangular hole. is this anything to worry about? will the new piston have less compression? more power?

edit: and the old one also had 2 small pin holes.

edit 2: and another question. the exhaust on the back of my dirt bike (not the big expansion chamber) says stainless steel, and it has yellow letters and these big red numbers. letters/numbers are all scratched up so i don't know exactly. one of the red letters is a 3. could this be a silencer?

edit 3: 2002 yz125, athena, and idk the number

 

[arnego2]

question two: yes most likely. maybe a r304.
Post a a picture of those 2 pistons that could help.

 

[ellandoh]

bike model and year, piston manufacturer and part number will get you better responses