Hi Vanessa,

Thank you for contacting Mearto with your appraisal inquiry.
Ken Griffey Jr hit his 300th home run on Monday, April 13, 1998; he would go on to hit a career-total 630 runs before he retired in June 2010 after a 22 year career during which he played for the Seattle Mariners (1989-1999, 2009-2010), Cincinnati Reds (2000-2008) and Chicago White Sox (2008)
Based on the photos and information provided, and subject to examination, this is:

A vintage limited edition Ken Griffey Jr autographed 300 Homerun Jersey
made by Russell, signed 12/28 (probably 1998)
signed and dated in metallic silver pen on his number (Mariners 24)
size 48
#12 of 18 limited edition
accompanied by JSA letter of authenticity
CONDITION: appears to be in good condition
PROVENANCE: unknown
$1,200-1,500*
*represents a fair-market value for auction purposes; retail or asking price may vary.

Please let us know if you have additional items to appraise, and thank you again for using Mearto.