[roger0080]

Thanks for the replies.
I dug into this further.
I have looked at a 1972 GMC Parts Manual & 1972 Canadian GM Parts Manual.
The GMC manuals is fabulous. There is no question about what axle they are refering to. They explicitly label the part as HO52, HO72, Spicer 60, Spicer 44 etc.
The GMC manual also gives dimensions on the drums, hubs for 67-70. No 71 & 72 though. Bummer.

I tried to double check the backing plate and the wheel cylinders.
Here is what I found on that:
Backing plate and Wheel cylinders confirm each other:
1967-1972 C20, 1972 K20: all have the same backing plate and wheel cylinder part number
1967-1971 K20: all have the same backing plate and wheel cylinder part number

Hubs:
1967-1970 C20,K20: all have the same hub part number
1971-1972 C20,K20: all have the same hub part number

This is what the difference between 1967-1970 C20 and 1967-1970 K20 Drums are (72 GMC Part Manual):
1. Width of where the shoe pad makes contact with the drum
C20 is 3-7/64th's wide and K20 is 2-13/32nd's wide
2. Width of drum (from insides)
C20 is 4-15/16" and K20 is 4-23/32" deep

I cannot find any references that there is/was a 12x2 drum for 1972 K20's.
I believe 4x4Poet your K20 with the 11x2.5 drums is legitimate.
I think your 71K20 Suburban should have 12x2.5 drums (71 was an oddball year). They may look like 2" unless one is super diligent when it was apart.
Your 2WD wheel cylinders that fit your 72 K20 now makes sense based on the above.
Which means the symmetrical wheel cylinders are for the 12x2 drum backing plates and thus you can't mix the wheel cylinders and backing plates. I'm going to guess C20 is 3/16" brake line and K20 is 5/16" brake line.
IIRC, my 71 K20 12x2.5 drum backing plate had symmetrical wheel cylinders (if I understand that right) and 5/16" brake lines.

If 12" drums for 1971 (1972?) are not available, I see over at rockauto.com there are some 67-70 12x2 (1/2" stud holes) available. I wonder if one could drill out (carefully) the 1/2" holes to 9/16" holes ! ??
Last minute thought, 67-70 hub opening in the drum is 4-19/32nds. Have to check that against a 12x2 drum hub opening.

Hope this helps someone. If not, it's intresting reading!
-Roger