69-70 rear seat in 72 burb?

[Mom's '72 burb]

Ok, ok, so I picked up my sweet burb minus the rear seat (2nd row). I searched & searched AZ for one to throw in there & ended up finding a seat with brackets & belts from another site member who happens to live like 10 minutes from me.....the "L" shaped bars fit into the grooves & bolt up on the floor, but the brackets don't bolt up on the backside (up on the lip)...there hasn't been a rear seat in there for at least the last 18 years, so I'm wondering if the owner before me had modified the bolt holes for some reason....or if a seat from a '72 has to go into a '72?...I crawled underneath and there are actual holes in the crossmember, with brackets going through them to bolt the seat to...am I chevy retarded or was this burb modified long before I got it? Any suggestions, help, anything? Please and thank you....