66’ GMC 4WD Frame Question

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This is a 1966 GMC 4WD K10 Suburban. The photos below are of the rear frame where the suburban frame extension is riveted to the 1 ton frame used for the 4WD model.

Question is the square cut off tubing ( U shape) on the under side of the frame original to 4WD Suburban? Note: 4WD Suburban frames are different then 2WD frames. It is high lighted in yellow. If so what purpose? (spare tire, gas tank) Any photos of it complete. Alternate, it is not factory original, possible some type of support for aftermarket bumper. Will do a more detailed review of this feature, welds, cuts, position on frame, etc.

NOTE:The last photo shows on 1966 GMC Suburban 4WD where the the Suburban frame extension connect with the 1 ton frame. The suburban extension appears to be some what similar to 1/2 ton 2WD rear frame area. There are some difference, the upper and lower frame flanges are wider on 4WD extension. Will plan to post photos of differences.