John wrote:
As far as I know, they were first used in formal invitations (to weddings,
funerals, dances, etc.) in the corner of the card, as both shorter and
more fancy than the older phrase "The favor of your reply is requested".
This is correct. The practice dates from the end of the nineteenth century.
At that time, transmission of text on long distances was with telegraphic systems, where texts had to be short because they were expensive, and because the available bandwidths were very limited to support many customers, notably for long distance and international communications.
I would not be surprized if this acronym was defined in some internationally accepted set of abbreviations used by telegraphists, so that their clients became exposed to these acronyms when reading telegrams received from their local post office that did not take the time to reconvert these acronyms to full words...
I have read some articles about the existence in telegraphic standards of such list of abbreviations. Isn't there a remaining, possibly deprecated, ISO standard about them? (For example there has existed the 5-bit system, because it was important to limit the available charset, and to limit the bandwidth required to transmit messages, at a time were searches on data compression was not as advanced and successful as today, and the computing resources or human capabilities to decode complex compression schemes would have been too much expensive or impossible to satisfy on a large scale).
