Early Divided Back Postcard in Burma

Britain first allowed this practice in 1902, with France and Germany following suit in 1905. The USA adopted it on March 1, 1907. The UPU officially endorsed the use of divided back postcards at the Rome congress in 1906, effective from October 1, 1907, in most member countries. Prior to this, there was no universal recognition, leading to various bilateral arrangements between member countries from the 1897 New York congress to the 1906 Rome congress.

Regarding to the inland Postcards, the Indian Postal Guide Oct 1907 stated that

In the case of postcards of private manufacture, however, the address side may also be used for communications, advertisements, engravings, etc., provided that a space not less than half the address-side, marked off vertically, is set apart on the right-hand side, solely for the address of the addressee, the postage stamp and the official marks of the Post Office.

Also the regulation allowed the postcards produced by private manufactures 

Cards of private manufacture may be sent as postcards, provided

1. that they are not larger than 5¼ x 3½ inches, or smaller than 4¾ x 3 inches, and 
2. that they are neither thinner nor more flexible than the inland postcards issued by the Post Office.

Regarding to the foreign postcards, Clause 159 stated that

Cards of private manufacture, with or without adhesive stamps affixed to them in payment of the postage, may be sent as postcards to any Union country: provided that, as regards size, they are not more than 5½ by 3½ inches or less than 4 by 2¾ inches, and that, as regards substance, they are made of paste-board or of paper neither thinner nor more flexible than the international postcards issued by the Indian Post Office.

Clause 160 stated that

Apart from heading, nothing may be written, printed, or otherwise impressed on the address-side of a postcard, with the exception of - 

• the name and address of the addressee;
• such postal terms as "Registered" or "Acknowledgement Due";
• the name and address of the sender

Provided that in the case of a postcard addressed to any Union country this restriction shall not be enforced when the following conditions are fulfilled, namely:-

(i) that a space is set apart - on the right-hand portion of the address-side of the card - for the address of the addressee, the postage stamp or stamps, the postal term referred to above, and the official marks of the Post Office, and

(ii) that this space is not less than half the address-side if the card, and is marked off by means of a straight line drawn vertically across the card.

Explanation - Under provisions (i) and (ii) above, the remaining portion of the address-side of the card may be utilized by the sender for written, printed, or impressed matter, including personal communications, engravings, advertisements, etc.  This concession is allowed only in the case of the official international postcards issued by the Post Office and of postcards of private manufacture, and does not extend to the official inland postcards issued by the Post Office when used for the foreign post, the available space on the address-side of the letter cards being sufficient only for a clear address, the additional postage stamp or stamps to be affixed and the official marks of the Post Office.

 In Burma, the earliest recorded use of a divided back postcard is dated January 15, 1906, published by Watt & Skeen.  However, the Watt & Skeen card explicitly states "FOR INLAND USE ONLY," highlighting the absence of international agreements at the time. Nevertheless, the card successfully reached England, possibly facilitated by Burma's status as part of the British Empire during that period.  The example shown below is from 20 Jan 1906.

20 Jan 1906, Divided Back Postcard published by Watt & Skeen

Another example, sent on April 26, 1906, from Rangoon to England and published by P.A. Klier using the color collotype method, represents one of the earliest instances of divided back usage. Klier's card lacks any notice restricting its use to inland destinations.  These cards precede the implementation of the UPU regulation.  

26 April 1906, Divided Back Postcard published by P.A. Klier

As of now, the earliest recorded divided back postcard usage dates back to 1906. While there might be potential instances from 1905, no cards of this nature have been discovered thus far.