Thank you so much for this. Gill-edge serration -- I assume you mean the edges of the caps in older mushrooms are serrated, while younger mushrooms have straight edges in L. tigrinis, correct? I've taken a few additional photos, please do see if these clarify? The first is the larger and older mushroom, images 2 and 3 are of the younger.
As for growing near river banks -- these weren't at all in a riverine area, but we have had a LOT of rain and localized water-pooling/ flooding in recent weeks & so it's possible that certain micro-ecosystems have emerged with the season maybe? We do get most wild mushroom growth right after the monsoons & I've certainly not seen these mushrooms around here before.
Pollen print wound up white-tending-to-yellowish, by the way.