The very important thing is: your duties described in the reference letter should match to the NOC description.
This is common reason for rejection: not correspondence between official NOC description and reference letter.
As for NOC, before (one year ago) they didn't ask to put NOC in the reference letter. My reference letter didn't have NOC, and I got medicals already.
However, now they ask it to put NOC (IF KNOWN)... I think it is better to put, just for the case.