Metal tile with slot in hallway of old university building. Standard floor tile size. Any ideas ?

Metal tile with slot in hallway of old university building. Standard floor tile size. Any ideas ?
Answer: It’s a dust collector for a built in vacuum system. You dry mop the halls then clean the dry mop using the vacuum at this slot. It used to have a lid that fit the long slot and sealed it. The round part is a hookup for a vacuum hose. My high school had them and the janitors used it.