Metal Cap Block
A Metal Cap Block is an object which disappears and turns into a Metal Cap when Mario hits it. It respawns after a certain period of time.
When the Metal Cap Block is ground pounded or attacked in some way, it is set to be intangible and invisible, rather than unloading. It's internal timer is set to 0 and it spawns a Metal Cap. When the timer reaches 301, it is reset to 0 and, on the next frame, the Metal Cap Block respawns.