Actually it would seem the type of fish may be red herrings
. I am not sure whether the type of fish are pertinant as often the motif in material culture and of course weapons becomes stylized in representative allegory. As Jens has mentioned, beyond the Hindu representations of fish which I mentioned in what I wrote back in 2007 (Pandya) and the Matsya incarnation of Vishnu, the fish was indeed a dynastic symbol of the nawabs of Awadh (Oudh). In the attached illustration of the silver rupee of Awadh (c.1814-15)there is a stylized fish, which does not however, resemble the examples seen on this katar. Still, in the sense of stylization, the actual image chosen by the artisan is of course his choice and based on personal interpretation.
It remains interesting to note the various applications of the fish in symbolism, and thier auspicious meaning in different contexts. While the elements of this katar reflect characteristics of Tanjore katars, Elgood does note that Hindu designs in these katars did influence examples in Golconda/Hyderabad, and from there to Lucknow. Lucknow was the capital of Oudh, so although admittedly tenuous , it is tempting to consider the possibility of this katar being from these Mughal regions. The representation of architectural designs in Indian weapons is a topic worthy of considerably more discussion, and in the attached illustration of the winter diwan (assembly) of a Mughal nawab some interesting similarities to the hilts of some katars and the arches surrounding in the room are seen.
Just more observations to discuss further.
All best regards,
Jim