Does the IRS letter suggest to which years it should be carried back?

Best place to start your research is with the Pub 536 for 2017.  It lists some exceptions to the two-year rule, which probably don't apply to your case.  You might also want to check the 2018 and 2019 editions because Congress keeps making retroactive changes to NOL rules.  

https://www.irs.gov/pub/irs-prior/p536--2017.pdf