Learn English – this letter/symbol called

Logogram

A logogram, or logograph, is a grapheme which represents a word or a morpheme (the smallest meaningful unit of language). This stands in contrast to phonograms, which represent phonemes (speech sounds) or combinations of phonemes, and determinatives, which mark semantic categories.

Logograms are commonly known also as "ideograms". Strictly speaking, however, ideograms represent ideas directly rather than words and morphemes...

Other dictionaries seem to equate ideograms with logograms. Dictionay.com World English Dictionary for example:

Ideogram:

1. a sign or symbol, used in such writing systems as those of China or Japan, that directly represents a concept, idea, or thing rather than a word or set of words for it

2. any graphic sign or symbol, such as %, @, &, etc.

From the examples in this article on Medieval Missives & Letter Writing in that period it appears to have been customary to address the recipient indirectly (i.e. in the third person) with the phrase:

To my…

or

To his…

The interested reader can consult the page himself, so I only offer one example:

12th century England: mid-October 1170 From John of Salisbury to Canterbury during the exile of Thomas Becket, original in Latin

[address] To William Brito, sub-prior, Robert, sacrist, and the obedientiaries of Christ Church, Canterbury

To his dear friends and brothers, William the sub-prior, Robert the sacrist, and the others who have charge in Canterbury's holy church, their John: greeting - and do you make wise provision for the future. ... Think of yourselves and of us, in love, and farewell.

Note that this use of “To…” is distinct from that used in the address.