The different early Ford axles are easy to identify by referring to the picture below, which was scanned from an old magazine article by Neal East.
1928-1931 Model A axles have a uniform curve from end to end. The spring perch bosses are 2 1/4 inches.
1932 axles were the first Ford axles to be made with a slight drop. These axles are often referred to as 1932 heavy axles. They are unique among early Ford axles in having a wider recess at the ends, and a slightly raised area on the front and back of the spring perch bosses in the shape of an I-beam. The spring perch bosses are two inches.
1933-1936 axles have narrower recesses at the ends and along the center compared to the ’32 axles. The spring perch bosses are two inches.
1937 V8-60 axles the only early Ford axles that are hollow forgings. They are flat along the front and back, so their cross-section resembles an oval track with short straightaways. They are relatively lightweight and not recommended for heavier cars. Their spring perch bosses are 2 1/4 inches.