Just got all the information straight from a tech at Wells Gardner. He agreed that the interface board is the problem. If you have this monitor with revisions A,B,or C, the light gun will not work. The revision D interface board is just the old boards with slight mods done to it, and the revision E interface board has all of the fixes already done to it at the factory. By the way, if you are looking at the backside of your monitor chassis, this interface board is located on the right hand side. To check which revision you have, you will need to unscrew two bolts that are attached to a bracket that holds the board onto the chassis. You will then be able to turn the board around and locate the info that you need. When I went to the Wells Gardner site I was able to order the part that I needed to fix my problem. The part is PN279200 Neck Board (includes interface board PI279201 with revision E) Part was $50 + shipping.
Hope everyone can benefit from this info.