At this stage, if you can't get it replaced for a new one, you will have to buy it.
I think they did a silent redesign for the Original Smart Keyboard. If you can give them an accurate serial number on your keyboard at the Mac Store they might give you a replacement.
Search the thread for instructions on how to get the cleanest image of your serial number since it's now down hard to read due to usage on flat surfaces. I noticed that they made a huge effort to make the serial number on the new keyboards very dark and much easier to read.
In a nutshell use your iPhone Video feature , shoot in 4K with the flash on, and you should be able to get the clearest image the serial. The still photos just don't cut it.
Good luck!
MJW