Need Help With Aerogrammes

The problem is that no descriptive list of the locations of Indian Experimental/Temporary POs has yet been published.
Most of the related official administrative records are at the Library of the Postal Training Center, Saharanpur and it is not accessible to the general public.

The only way to briadly determine the location of the Experimental POs is from the content of the mail which has just been the case here i.e. the location of Experimental PO N-2118 in July 1956 was in Mahu (22.55°N 75.7667°E), now in Indaur district of Madhya Pradesh.

The use of the alphanumeical codes to designate a particular Experimental PO in India began following the Post Office Circular No.40 dt. July 16,1883 issued by Frederick Rogers Hogg, Director General of Post Office in India (1870–71, 1873–75 and 1879–87 with intermittent breaks).

The alphabets in the code denoted the initial of the Circle HQ and these alphabet codes were first allotted in February 1873 following the Indian Postal Manual 1873; Section II: Stamps and Seals; Chapter VIII: Stock; Para.36: Initial Letters Indicating Circles; p.80, when the letter N (denoting the initial of the Circle HQ Nagpur) was allotted to Central Provinces Circle (created in April 1867) as well as to Experimental POs established for Expeditionary Forces.

Nagpur remained the HQ of Central Provinces Circle from April 1867 to April 1909 when the Circle was renamed as Central Circle which remained so even after 1947.

Little bit of trivia: The British Indian PO of Mahu was established in 1818 to cater the personnels of the Army Cantonment in Mahu which was inside the territorial limits of the feudatory state of Indaur.

Incidentally, Indaur state maintained a very cheap but efficient intrastate public and official postal system from 1873 which followed the Brahmani Dak postal system prevalent there from 1829.

After the enforcemenet of the Indaur–British India Postal Treaty 1878, Indaur State Post established 7 post offices in Mahu viz., Mahu, Mahu Barrack, Mahubazar, Mahu Branch, Mahu Divisional Headquarter, Mahu Native Infantry and Mahu Railway Station, all under 4th Circle of Indaur.

Excellent information, as usual. Thanks Joy.

The British aerogramme was issued in 1954, likely Kessler #13. This should then have the center folding line dots pointing toward "s" of "First Fold Here". Now there was a variant issued late 1954. There were more variants issued in 1955 but the cancel confirms the date of issue of this one.

The Angola one was indeed issued in 1957, Kessler #19.

Kessler's is an air letter/aerogramme catalog, last formally updated in 1968 with a third looseleaf volume. It was later updated for a time via the newsletter/new issue sales sheet The Airmail Entire Truth.