For those who cry fake: apparently letters from branches to HQ do not carry the letterhead.
See this example: letter from branch Australia to HQ:
Letterhead
Ending:
Source: official evidence of Royal Commission: https://www.childabuseroyalcommission.gov.au/sites/default/files/WAT.0004.001.0010.pdf
https://www.childabuseroyalcommission.gov.au/sites/default/files/WAT.0004.001.0014.pdf
https://www.childabuseroyalcommission.gov.au/sites/default/files/WAT.0012.001.0009.pdf
A different example:
https://www.childabuseroyalcommission.gov.au/sites/default/files/WAT.0012.001.0011.pdf
Even HQ doesn't carry a Watchtower letterhead when writing to branches:
https://www.childabuseroyalcommission.gov.au/sites/default/files/WAT.0012.001.0016.pdf
Yes I see it still looks different than the letter discussed in this topic, but at least the missing Watchtower letterhead doesn't mean it's a fake.