I looked, but TBH, not sure what to make of "genuine" and "OEM" claims for a battery for a 16–year-old device (or 10-year-old if you count compatible Lumias). Descriptions usually do not mention manufacture dates either.

> not sure what to make of "genuine" and "OEM" claims for a battery for a 16–year-old device

They're lies, but the batteries work. There were Chinese lines manufacturing knockoff BL-5Js, and there may still be one or two, or just a bunch of crates filled with old ones. Source: still use an N900, but just for podcasts.

> 10-year-old if you count compatible Lumias

Worse, those Lumia BL-5Js aren't actually compatible. The slots cut into the battery aren't wide enough to fit into the N900. Unless you're willing to cut apart the battery itself, they're useless.