Tail-bone.

I landed hard on mine once when I slipped and fell on a hike, and it bothered me for 5 years. The OP may have an old injury, anomaly/deformity, or who knows what.

I think his assumption about the more pronounced ridge on the saddle is a likely explanation.

Based on this I am going to suggest checking out mountain bike saddles.

I recently got this one, which fits my 120/125mm B17-friendly arse:

https://www.sq-lab.com/shop/en/Saddl...ve-active.html

They come in a bunch of different sizes. Mine was

1x 611 Ergowave Active S-Tube Saddle - 14cm #1896 for $169.99 each (sku: 1896)

Their US website is https://sqlab-usa.com