STEM OPT Extension Documentation

[Jun2580 27]

I am working under OPT, and since I graduated in a STEM major, I am collecting documents to apply for STEM OPT Extension.

In my school website, it lists the following as a few required documents to mail to USCIS,  

1) STEM OPT I-20 (The new one issued for STEM OPT)

 

But many other websites from other schools list 

1) ALL I-20s ever issued to you including the ones from other school you went to, the current one for OPT, and the new one issued for STEM OPT)

 

Which is true? Do I have to mail ALL i20s? In USCIS's website, it only says 

 

 

Form I-20, Certificate of Eligibility for Nonimmigrant Student Status, endorsed by your DSO on or after May 10, 2016

 

Another question is, do I need to submit Employment Letter or Offer Letter from my employer?

USCIS's website doesn't list that, nor my school's website. But many other school websites have listed that too, and I am confused.

 

Can someone, especially those who got granted STEM OPT Extension, please tell me what forms you submitted, or whether I need (1) All i-20s issued to me, OR just the STEM OPT i-20, and (2) an employment letter?

Thank you!