TQ Parcelpostguy and rod222 for posting.
Since, there is hardly any interest, here is the answer.
There are 6 Indian markings on this 1899 Shediac, Canada–Chennai cover, 1 postmark and 5 manuscript marks.

1. The blank elliptical mark is actually a registration postmark (Proud type R3/Dovey and Bottrill type 3) (recorded used Apr 13,1891–Aug 25,1902) applied at the Mumbai–Adan Sea Post Office on board of Peninsular and Oriental Steam Navigation Company mail steamer SS Massilia by the Set B Sorters.
This was generally impressed on incoming registered mail by the Sea PO which was designated as an Office of Foreign Exchange (OFE).
An OFE was the frontier or last Br. Indian post office thru which outgoing and incoming postal articles to/from foreign contries (as well foreign settlements in India viz., Índia Portuguêsa and l'Inde Française) passed.
The OFE had the duty to handle foreign postal articles for dispatch or receipt, keeping their accounts and conducting official correspondences relating to these matters between the two involved countries.
Initially, 4 OFEs were established in India in Chennai, Kolkata, Mumbai and Adan.
Later OFEs were opened in Nagappattinam, Tuttukkudi and in Yangon and Karachi.
2. The number 1254 in blue pencil, put in manuscript inside the Proud type R3/Dovey and Bottrill type 3 mark is the docket number used for keeping account of incoming registered articles, entered by one of the Set B sorters of the Sea PO on board of SS Massilia.
3. The
2 mark in red in manuscript is the beat number of the delivery mailman of Tiruvallikkeni Sub PO under the delivery domain of which the address came.
It may be mentioned that the nearest post office to the delivery address, actually was Cheppakkam SPO but since it did not have any delivery facility then, the delivery of mail at the Cheppakkam area was done by Tiruvallikkeni SPO.
4–5. The Mayilappur annotations in violet pencil were most likely entered at Mumbai OFE.
6. The addition of specific locality Mayilappur in the city of Chennai in the address, was also most likely done in Mumbai because the address was served by Tiruvallikkeni SPO, not Mayilappur SPO and this type of mistake was least likely to be made at a post office in Chennai like Chennai OFE/Chennai GPO which handled this cover.