What is the function of grooves on cylinder wall. They are only 1/2 MILLIMETRE deep.?

It's a 48cc two-stroke motor. The pair of parallel grooves are about 2 or 3 cm apart, appear to be factory machined, and are on the left, midway between the intake/transfer port & the exhaust port. Problem is, they are (were) burred in places and have left marks on the two piston rings.