My understanding is that it's actually ~40mm (or even 42mm). But of course that's not a standard focal length, so either 50mm or 35mm is closest.

EDIT: Ah yes, I remembered correctly https://theonlinephotographer.typepad.com/the_online_photogr...

~40mm isn't that unusual. I have a 40mm pancake lens for my full-frame DSLR and I've had rangefinders with about that fixed focal length and it was pretty common as the low-end of a lot of digital point and shoots.

It's true. But it's also not nearly as common as 50mm (or even 35mm) on DSLRs

Certainly. Before zooms became common, 50mm was absolutely the standard "kit" lens for an SLR. Personally, I always preferred something a bit wider for day-to-day. Of course, most people generally use zooms these days.

35 is 50 on a cropped sensor. But I also feel like my vision is closer to 35mm full frame (23 aps-c).

With no crop factor, 50mm is pretty close to what's "in frame" for me, with my glasses, and 35mm is pretty close to what's "in frame" for my total field of vision. You'd have to defocus pretty hard to make a 35mm lens match what my uncorrected eyes actually see, though...

This conversion doesn't actually make any technical sense. Yes, you can calculate the field of view, but the depth of field, the angles of sight lines etc. are that of a 35mm lens.

For an intuitive way to visualize this: look at a few dolly zooms and think about what you are seeing in terms of geometry.