These early '50's G.M.C.'s tandems are very similar to a Hendrickson set-up.The front axle is powered (directly) through a transfer case. The rear axle is powered by vacuum,(when needed),through a vacuum switch,identical to a S/A 2-speed. It's marked "Wet/Dry pavement",instead of "Hi/Lo". The rear axle is "free",on the "Dry Pavement" setting,and is engaged when shifted to "Wet Pavement". I believe top-speed of 35 M.P.H.,is recommended (by Mfr).,for axle-inter-lock. Power is supplied by a driveshaft that sets at an angle,from the transfer case,ABOVE the front drive axle,to the rear axle.
http://s1181.photobucket.com/user/willybickey/media/7-3-2013/MOV00219.mp4.html?sort=3&o=1
