Suburban interior panels--B-pillar: Screws AND welded? Why??

[Brad54]

I've always wondered this... my '61 Suburban has interior metal trim panels, and the B-pillar panel behind the doors is attached with large, domed-head Phillips screws... AND it's welded at the door opening, behind the weather stripping.
WHY?!

I'd like to remove that panel and fabricate upper and lower seat belt anchor points, and I'd like to get in there and install bear claw door latches.

Anyone know what the thinking was behind screws AND welding, rather than just welding?

Has anyone ever removed those panels and gone into this area?

(and does anyone have one or two of those big screws? I think I stripped the blind nut on one of them and will have to cut the head off the remove the panel. )
-Brad