I ordered these battery from Flashlightgo. Here are the measurements, capacity and the look of K10. I used Plastic Caliper for measure length of the battery.

Charger I use: Vapcell BL4, XTAR VC8S

Button top: K10 length is 0.3 mm shorter than H10

Flat top: K10 length is 0.9 mm longer

The K10 button is very short and slightly wider compare to pointy button on H10, less pressure on the spring.

The positive top is different showing K10 maybe made by a different company, the warping on the negative side of flat top K10 is more than I like, a bit inconsistent.

Capacity is more than rated similar to later H10 variant.

Both flat and button top work fine on D3AA, I use them for a few days and so far no issue. I will test on other lights and update here.