Unused Objects.

SuperMarioBrosBrownBall

An object that Mario can climb up or down like a vine. It makes a "buzzing" sound as you climb, as though Mario is repeatedly hitting his head on something. It can still be placed in-game with a level editor. Although it appears as a brown flagpole ball, the actual 16×16 metatile that is used is unique to this object.

Iink here : https://tcrf.net/Super_Mario_Bros.

I found that objects is actually FlagPole's top with object value of 0x31 containing at offset 0x0016.

Here is my tweet about it. https://twitter.com/haseeb_heaven/status/831679215783120897

oh yeaaah, i used that in one of my smb1 hacks.

Yes it can be placed with level editor.