Another victim of IRS turning on name control matching this year.  What a great year for them to force people to paper file 1041s.

I've heard trial and error might be the best approach.  You can try calling but I've heard on other lists that the "help" line isn't allowed to give out this info over the phone.  The best you might get is someone who will confirm it if you guess right.

Some resources I'm stealing from another forum (but the conclusion there was that the IRS is inconsistent and their name control rules have changed over time so it depends on when the trust was created):

https://www.irs.gov/businesses/corporations/using-the-correct-name-control-in-e-filing-corporate-tax...

Q6/A6 has the phone number.

https://www.irs.gov/pub/irs-pdf/p4163.pdf

Start reading at the very bottom of page 37 (jump to PDF page 49, the page numbering is off).  Page 41 talks about trusts.

Are we having fun yet?

Rick