do you know what kind it is?

it looks very similar to mine which is 'Philip Seiden'. but Seiden has a bit more dark-red-brown in center. perhaps, yours can redden too if given enough bright light?

Looks very similar to 'Fireworks'

I lose many Rexes, I managed to self Antonelli's Rex before I lost it, stored the seeds, finally put them down. There were four variations in the resulting plants, I kept only this one which did look somewhat different. I have 4 of the Japanese Begonia Society books and found a plant pictured in one of them which does resemble my plant somewhat. Their plant looks as though it has a pure white surface with small black marking near the sinus. The name is B. 'Madonna' and it is well deserving of that name. I wonder if it has ever been shared or sold.

Rexes are one of my favorite type of begonias - so colorful that it needs no flowers (other than furthering its genetics).

Here are a few at Harmony Foliage when I visited last year.

Hc mcdole, just seeing these photos for the first time, awesome nursery you had visited, I love the plant in your first photo!