Firstly, as we saw in the Blog about the Garter, appointments are usually, though not exclusively, announced on the Saint's Day of the Order, 23rd April, St George's Day. Secondly, it is unusual for the Monarch to appoint their heir's consort to the Order. The first appointment in modern times was Queen Alexandra, Consort of HM King Edward VII, but she had to wait until her husband ascended the Throne before he made her a Royal Lady of the Order. Queen Mary and Queen Elizabeth (later The Queen Mother) were similarly appointed by their husbands. However, if precedent there is, The late Prince Philip, Duke of Edinburgh, was appointed to the Order by his father-in-law, HM King George VI on 19th November 1947, the day before he married the then Princess Elizabeth (the present Queen). This was just over a week after his bride, just to show that she took precedence. Both, however, were formally installed the following year at the Garter Day ceremony which Camilla will, presumably, officially go through this June. This may be to take the pressure off The Prince of Wales when he comes to the Throne as, in many circles, Camilla is still a controversial figure.
The Communication specifies 'Royal Lady' and therefore one currently assumes that Camilla has the post-nominal letters LG. Debrett's, "the modern authority on all matters of etiquette, social occasions, people of distinction and fine style", states in comparison, that HRH The Princess Royal and HRH Princess Alexandra, the Hon Lady Ogilvy, are both Royal Knights Companions and they bear the post-nominal letters KG (not LG).
There is no limit to the number of Royal or Stranger Knights or Ladies, but Companions are restricted to 24 in total. Unfortunately, there have recently been a number of deaths and the appointment of Baroness Amos and Tony Blair still only brings the total number of Companions to 21. There may, therefore, be further appointments on 21st April this year, especially as this is Jubilee Year.