Rumor is the brackets you posted work for my truck. They don't, but can I ask what you installed them on?
For what it's worth, here's some info. about the parts. Incidentally, my frame is a 5.3/10" bolt pattern.
I believe all the rest are a 5" bolt pattern.
http://www.gmpartswiki.com/getpage?pageid=22606
3885614 - Chrome Rear Bumper Bar. 67-72 C-10-20 (Sub, Fleetside) Blazer
3890391-2 - Inner Brackets. 71-72 Ser. 10-30 (Stepside, Fleetside) and more.
Reads excludes leaf springs. Found an NOS on the Chevy site, it’s the bracket folks call “coil spring” inner brackets. NOTE: The same part was used for the front bumper on 67-70 "pick-ups" (excludes 4x4)
3952089-90 - Inner Brackets. 69-72 Ser. 10-20 (4WD) (Pick-up)
Reads Rr. Bumper, which is rear bumper. No web reference found. I believe these are what folks call “4x4” inner brackets (What Richard's link shows).
3891985-86 - Inner Brackets. 68-72 SWB 2WD with leaf springs.
No reference found on the web for this bracket. Not surprising. My Chevy is an outcast
3891987-88 - Outer Brackets. 68-72 SWB 2WD with leaf springs.
My outer brackets are an oddity as well. Ironically there are quite a few hits on the web for this outer bracket, reproduction. They're referencing the wrong part number. The brackets they're selling fit everything but a SWB 2WD with leaf springs.
3892477-78 - Outer Brackets. 71-72 and more. Ser. 10-30 (Conv., 4WD), Blazer (Stepside, Fleetside) and more. Reads “Aux. leaf springs excluded”. Can’t say what an “Aux.” leaf spring is, but OK. This is the common outer bracket, works on everything but a SWB 2WD with leaf springs. Found an NOS on the Chevy site, looks like the typical outer bracket. Other than that nothing found on the web.