Denny, As far as I can determine, there were two types of sending units used on AD trucks. The earlier style, ( I believe '49 and '50 ) had a gear arrangement on the float arm with the electricals mounted in a housing externally on the unit. The gears transferred the up and down movement of the float arm into a rotation of a rod that ran to the actual guts in the housing. Kinda hard to explain, I've got an old one around here somewhere. If I find it, I'll send a pic.
In '51, they went to a submerged type, such as what you have, only it's all metal construction.
I believe they changed because of a propensity of the gears to corrode and freeze up on the earlier style, plus they were probably cheaper to make.
Bob
