for info: http://www.tractordata.com/farm-tractors/004/9/8/4988-mccormick-deering-o-4.html
IH commissioned an industrial designer, Raymond Loewy, to give the new Farmall general-purpose tractors a sleek new streamlined look. Designed for small-to-medium size American farms, IH's new machines offered a wider variety of capabilities, engines, and equipment options.
IH took care to produce a model for almost every farm and every need. Sales took off, and letter-series production did not end until 1954. Overall, the Farmall 'letter' series, well built and affordable, became not only a defining product line in IH history, but an iconic symbol of the prototypical American small-farm tractor. Many machines (especially the two largest models, the H and M) are still in operation on farms today.