I am still using 231 from metal cans myself. The number on the bottom is the lot number, so there is nothing to decipher other than trying to match lots when buying more than one can at a time.

I don't recall right off hand when they switched to the smaller rectangular plastic cans, but its's been a while now. As long as the seal under the cap of the cans you are looking at isn't broken, the powder should be good for a very long time. I have several cans of older powders on the shelf that are at least 20 years old, and they are just as good as the day I bought them according to the look, smell, and the chrono.