Объясните минус. Такой вариант есть в пособии Company to Company: New approach to Business correspondence. Cambridge University Oress

минус, но мой, но мне "of" в таком контексте ни разу не встречался. зато "letter from 6 July 2016" встречается в официальной переписке часто

Но если Вам не встречался такой вариант, это не значит, что это неверно. Я сама употребляю dated, просто привела здесь как альтернативу.

http://forum.wordreference.com/threads/please-acknowledge-receipt-of-this-letter-email.2404567/

Елена, не спорю.

I have to say that I've never seen OF used like this in letters. It may be an alternative, but it's not a good one.

Uly, but it does exist in British English at least

It exists in Am. English too, apparently, but I wouldn't recommend it to anyone. It sounds awkward. Most of my work as a translator consists of proofreading and editing correspondence and I would immediately change this to DATED if I came across it in a letter. To wit, I've never once seen this in all my years. But I googled it and you're right, it exists.