What do the codes on the sides of headset bearings mean?

I’ve just taken my headset on my gravel/commuter bike to bits as it’s feeling a bit rattly, lo and behold after five years the bottom bearing is a bit knackered.

It’s a tapered headset, an FSA C40 ACB apparently.

The top bearing has  TH MR121 1 1/8 45°*45° B8 written on the side of it.

The bottom one has  TH MR127 1.5 36°*45° A8 written.

I get that the 1.5 and 1 1/8 refer to the internal diameters, and that the degrees are the taper, but what do the other numbers and letters mean?

What does C40 ACB mean also?

Can I just chuck in the right sized bearings or do I have to buy specific headset ones to replace them?

Thanks.