Charlie,
Hi. The 1932 Ford marked the introducion of the famous flathead V-8 engine. Baeing that 1932 was the first year (1) of the eight (8) cylinder engine it was trus given that Model 18 serial prefix.
On the other hand, the "improved" four cylinder model was viewed as an advanced Model A type offering. Following the early history of the auto industry, where model changes were identified by progression of the letters, A,B,C, etc., then it only natural that the improved 1932 Fords with the four cylinder engine would be identified by Ford as the Model B.
I sure hope all this history stuff makes sense to you but that is the reason for the Model 18 and Model B. Take care.
Keep V-8ing,
Dale