You asked me for some further assurance that the Peace Treaty which the British Government now offer, contains nothing that interfered with the complete liberty of Afghanistan in internal or external matters.
My friend, if you will read the Treaty carefully you will see that there is no such interference with the liberty of Afghanistan. ... the said Treaty and this letter leave Afghanistan officially free and independent in its internal and external affairs.
Moreover, this war has cancelled all previous Treaties.
I have come across many sources claiming that the Treaty of Rawalpindi was signed on August 19, 1919 (e.g. The History of Afghanistan), but this is incorrect.
I have also come across some sources claiming that King Amanullah Khan declared independence on August 19, 1919 (e.g. Afghan Embassy in the US). It is certainly possible that independence was only formally declared 11 days after the signing of the Treaty of Rawalpindi. However, I have tried but am unable to find any solid confirmation of this and fear that this is merely the above mistake repeated.
There is also one webpage that states:
19 Aug 1919 Independence celebrated (proclaimed 13 Apr 1919).
(One possibility I have considered is that there is some calendar thing going on, similar to the October vs November Revolution. However, I don't know enough about this -- and in particular the Muslim calendar -- to pursue this much further.)