On letterwriting, etiquette

I have so far been using the initial of my first name, M, wax seal to close my envelopes. However, is it proper? Should I be using my surname (my married name)? This question has led me to etiquette, tradition, good form written in the late 19th century and early 20th century.  The answer is that I should use the initial of his surname. The initial of my first name never changes, but the surname might and already has. So, I am content to use M. How many of you when telephoning someone, say, "It's me!" ? 

However, what is good form way back when seems to me to be artificial and inequal. There are rules. Who came up with them? There are ways to address the envelope. It is not etiquette to use Mr for a gentleman, but for tradesmen & mechanics. I rarely use titles, though if I am writing to someone who has a PhD, or is a doctor of medicine, I have used the form Dr to them. I won't address the envelope to a married woman as Mrs [her husbands's initial] [husband's surname]. There are still a few people around who say that the woman is property of the husband. Although I have taken my husband's surname, I still identify with my first name, and its initial. Forum posts sometimes have me signing off with this initial. 

Perhaps these rules constrain the feeling of the word/letter, the individuality and uniqueness of each correspondence, the creativity; emotions weren't to be shown, stiff upper lip. I hope my written letters have character and soul to them. I want them to bring a smile upon the recipient's face, and joy. 

Some of rules of antiquated social letter writing, taken from Arthur Wentworth Hamilton Eaton's book in the Good Form series, "Letter-Writing Its Ethics and Etiquette with Remarks on the Proper Use of Monograms, Crests, and Seals" (1890) I break are:

• Lined papers must never be used for social correspondence, they are extremely bad form.

• There was a local fashion, some time ago, to use violet ink, but colored inks are never correct. 

• Using papers other than cream or white in colour

• Using anything other than the initial of my husband's surname (or crest if he had one) as a wax seal. 

• The use of wax in colours other than red or black.