Way back in the dawn of automobile manufacturing (and probably before) Fisher Coach was its own company building bodies of cars, wagons, etc for other companies that build the chassis and running gears, similar to Pinifarina in Europe, and half a dozen coach builders here in the US. Back in the 20's and 30's you could have a custom body contructed to be put on most any higher end chassis. Fisher plants were co located with the GM assembly plants but maintained a mostly separate organization. They were esentially subcontractors to GM assembling "Body Tubs" for GM to finish assembling into completed cars. GM paid Fisher for each Body Tub they assembled.
Before 1926 fisher was building bodies for just about anyone in Detroit that was building cars. Between 1916 and 1926 they had a capcity of 370000 bodies a year and they built for Abbot, Buick, Cadillac, Chalmers, Chandler, Chevrolet, Churchfield, Elmore, EMF, Ford, Herreshoff, Hudson, Krit, Oldsmobile, Packard, Regal, and Studebaker.
Back in 1926 Fisher Body was bought out by GM (they already owned 60% of Fishers Shares) and they became a separate Division within GM. GM complained that Fisher built their assembly plant too far away from GMs plants and were therby holding their production up, so they bought them and moved their plants next to GM assembly plants to cut down on delays. Fisher was paid on cost plus basis and allegedly had some inefficient processes that were increasing the prices of the bodies that GM had to pay. So GM felt it was in their best interest to buy Fisher body up, locate the assembly plants near theirs, and change their processes to be more efficient. Up until about the mid 70's they remained a separate division within GM but at that point they were completely absorbed and just became part of GM. Up until that time they maintained their own separate identity, organization and personel.