[justcuz]

I would guess it was for a heavy GVW package on the Suburban. The BOM may have been changed for GMC parts matchup since the rear axle is not common.
GM stopped using Napco conversions in 1960, unless it was a 3/4 ton suburban (8 Lug) or panel or 1 ton pickup.
If it was a Napco it would have had HO52 or HO72 front and rear axle housings with the dropout center sections like a 9 inch Ford. Someone posted up a picture of a 1 ton GMC panel truck that was a Napco showing the axles.
What you have is actually more unique than a Napco, not many Dana 60 semi-float rear axles were used in early GM products, especially a Suburban.
The most common modern use of the semi-float Dana 60 today is in light duty F-250's from the 80's.