What do new blocks look like in an earlier version of Minecraft?

I loaded two versions of Minecraft, 1.12.2 and 1.11.2, on my Minecraft server. I installed a plugin that allows players to join an earlier version of Minecraft. I put concrete powder in my inventory, and in Minecraft 1.11.2 it looked like Wool. I want to see if other items look like other stuff in earlier versions of Minecraft.

Solution 1:

They don't show at all. If you run a server that permits time travelling (logging in with lower versions), you can't utilize the features of the newer version and are limited to the older version's functionality and items.

Attempting to tell an older version about a newer item will only result in bugs or errors, or the older version not knowing what to do.