My key is 1x43-15K601-AD but I see on eBay keys that ending with
AE, AF, AG etc. so I wonder what is the difference and if FCC ID and rest
is the same, can I program the key with different letters to work with my
2004 Jag XJR?

 

Are you saying that no one of 60 peoples who red this post know what is the
differences or commonalities between Jags keys with different endings?

Does anyone wants to know why they are different?

 

Yes, I would like to know why the numbers/letters differ.

Were you able to program the new fob to your car?

 

Generally, the letters indicate the version of the part. "AE" would be newer than "AD" and so on. As far as I know, you can always use a newer revision than the original.

 

Yes, you can use the remotes that have the last letter farther down the alphabet than yours. Succeeding parts increment the last letter. Those prior to -AF are known to POSSIBLY loose the rolling code synchronization once in a while. Even if that happens, reprogramming the remotes restores the operation.

Cheers,