Hi Dwayne,
Differences in serial numbers by years across different models of mills is to be expected. Generally, more quantity of smaller mills like the #2 might be expected to be sold per annum compared to the larger #3. At least that sounds reasonable to me.
Follow Ken's (4gsr) good advice for looking for manuals ( and maybe parts) from Bourn & Koch.
I have an old original B & S Practical Treatise on Milling and Milling Machines in well used condition, but no mill to go with it, if you are interested in it. It is copyright 1939, printed in 1948. Contact me if you are interested in it. If you hover your cursor above my ugly picture on the left, a pop up window will open where you can start a "conversation." Contact me there.