The only thing common between them is their similar displacement.
Shivalik is the link between completely Soviet designed vessels and indigenously designed one; it's like an upgraded Talwar. And it's also the point from where we started to focus more on RCS reduction. Same Fregat, arm launcher, weird mast as Talwars but better ADS and significantly improved stealth, interestingly if the bow and mast is modified even a bit then Shivalik might be the most stealthy ship designed here. The silhouette is extremely simple
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Corvus is like 99% accurate with his explanation of merging...
Now comes P-15 Alpha and we move quite far from Soviet design. Most of the weapons and doctrines are still Soviet but the design has now changed. And this exact same layout is used again on Bravos. But even more interestingly, it's used in Nilgiris too albeit by shortening a bit. Take a P-15, select the area around second smokestack...
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...and delete it.
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And there you go, that's more or less what's Nilgiri is.
This is the reason I call them P-15 Charlie.