With the start of World War II, the veterinary medicine curriculum was changed from a four-year program to a three-year program. DVM students originally scheduled to graduate in 1944 graduated in the fall of 1943 so Iowa State has two Classes of 1943.
Subsequent classes (1944, 1945, 1946 and 1947) all began their DVM programs during World War II and were under the three-year program. The curriculum reverted back to a four-year program beginning with the Class of 1949, hence no Class of 1948.