The extension is used on 3800's with dual rear wheels to extend the front tread centerline. The 4-wheel versions like the 3800 pickup with 4 wheels instead of 6 don't need the extra tread width and the front hub us different. They also use wheels with a different offset.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!