Not sure, but to find out you could check the operators manual or shop manual, or drive it in both positions, you will defiantly feel the difference.
If you can't drive it yet jack up a wheel and turn the driveshaft some number of turns while you count turns of the wheel, switch and do again. The position that yields more turns of the wheel for the same number of driveshaft turns is the "fast" gear.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-