
The origin of this 1814 stampless letter is Merath (28.9833°N 77.7°E), now in Merath district of Uttar Pradesh.

And this is the Merath P
t. P
d. (Post Paid) oval dispatch mark [Giles 3 (U15)/Lowe 223] of Merath PO (est. 1800 but its first postmark is recorded from 1809) which is recorded being used from 1814 to 1837 in black.
What has happened here is the superimposition of the vertical stroke of
4 of the numeral
814 of the London mark over the left diagonal stroke of
M of the tripartite Merath postmark.
Here is the complete Merath P
t. P
d. oval black dispatch mark (Giles 3/Lowe 223) on a 1827 Merath letter which was auctioned as lot 152 of India Postal History The Soli F Contractor Collection in Spink Auction#1217 on December 16,1999 by Spink and Son Limited, London, England.

The vagaries of Anglicized corrupted Indian toponyms resulted different spellings of same placename.
The use of single 'E' in the Anglicized version of Merath was prevalent before 1837.
The use of double 'E' for Merath is first recorded in the List of Post Offices in B Presidency in 1837 but its first use in postmarks is recorded by Giles much later in 1845 in Merath
Overland Giles U59/Lowe 398 octagonal red transit mark.

This is Giles CG6/Lowe 263 red receipt cds of Kolkata GPO (est. April 1,1774) which was in use in 1808–15.
Incidentally, the cds only had the year digits
180 during its entire life of existence. The last digit/last 2 digits after 1810 were entered in manuscript, though here no year was written which was quite common practice.
References:
1. Handstruck Postage Stamps of India.
Derek Hammond Giles.
The Philatelic Society of India, Mumbai 1960
Appendix M: Governmenet Gazette Advertisement General Post Office March 12,1816; p.127
Catalog of Handstruck Postage Stamps of India
1(a)(iii) Kolkata General Post Office; p.177
1(c) The Upper and North West Provinces; p.198
2. Catalog of Handstruck Postage Stamps of India.
Derek Hammond Giles.
Christie's-Robson Lowe, Bournemouth, England 1989
Chapter VIII: Alphabetical Catalog of Post Offices