The "black painted steel bar that looks like it's supposed to go through the seatbelt buckle end " is part of your seatbelt anchoring system.
It does go through the lower portion of the buckle and your seatbelt with not function properly without it doing so. The purpose of the bar is to supply a "localize anchor point" meaning where ever the seat is adjusted, full forward or aft, the seatbelt buckle has a point close to the lowest portion of the hip. Since a seat can accommodate both a 4' X" to a 6'X" persons, without a moving anchor point the buckle cannot effectively accommodate all body types. The sliding buckle mount can.
Typically you can remove the front seats without also detaching this anchor by removing all four seat bolts, any wiring and then sliding the whole seat assembly forward until the inside buckle assembly comes off the steel bar you mentioned. Generally you do not have to unbolt this steel bar.
Hopefully this was helpful.
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