There is an English grammar problem which has been troubling me for a long period of time. The problem is with the construction "We refer to your letter dated XXX and addressed to YYY".
As per my knowledge, both of the "date" and "address" are acts which have been done, i.e. the date XXX has been written on the letter, and the name YYY has been written on the letter, and therefore, we should use past tense in the relative clause, and the full sentence should be "We refer to your letter which was dated XXX and [was] addressed to YYY". However, I have found that present tense can be used for the two past acts i.e. "date" and "address" (even they have been done), and the full sentence will become "We refer to your letter which is dated XXX and [is] addressed to YYY".
An example provided by the online Oxford Dictionary is "The document is dated Aug.24, 2004, but it has apparently just been put on the Web site very recently." (https://en.oxforddictionaries.com/definition/date)
Hence, I would like to know what proper tense should be used in the relative clause or for the two acts "date" and "address".