Through some research, I found the answer to my questions. Thanks to Sherry at Oliver Heritage for an article found in the Oct/Nov 2006 issue. Last Christmas I received the digital copies of the back issues of Oliver Heritage magazine. 

Below is the explanation of the Oliver 22 mowers. 

In 1948 the 22 was completely redesigned by the Shelbyville, IL plant and production for the 22-A began in Shelbyville. 1948 This was a more universal mower and had either a hand lever lift or hydraulic lift. It used a one-arm cast caster wheel support but there were two caster wheels. The gear case number on the 22-A is Z-425. The 22-A was a very popular mower and was sold with different hitch assemblies so it could be used with the other brands of tractors. 

This mower went through quite a few minor changes during production. The early caster wheel spring was a larger coil with just a couple wraps. In last 1950, this spring was replaced by a more efficient spring. It was a smaller spring with quite a few more coils to it. Also in 1950, the wheel spindle assembly which housed the wheel bearing was changed. Early models had a bolt on triangular shaped assembly while the new ones had an assembly that was welded on to the frame assembly. In the fall of 1951, the gear case cover was changed. The old case cover (Z342XA) had a long front nose that the PTO shield bolted to. This was prone to breakage so a new cover (Z6634) was used. This was a shorter cover but had a longer bolt on shield. 

The caster wheel support is the easiest way to identify which mower you have in the 22 series. The 22 used a single, round, forged support arm. The 22-A (above) used a single heavy cast support arm. The 22-B (below) used two steel support arms. 

Thanks for looking. 
John Schoenauer