Per IRS Instructions for Form 5329, page 5:

Line 4

If any amount on line 3 was a distribution from a SIMPLE IRA received within 2 years from the date you first participated in the SIMPLE IRA plan, you must multiply that amount by 25% instead of 10%. These distributions are included in boxes 1 and 2a of Form 1099-R and are designated with code S in box 7.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.