1734-aent/A vs 1734-aent/B

Few questions regarding Rockwell controls...

Does the "/A" or "/B" suffix mean series A or series B ?
Is there a hardware difference ? Can I replace a /A with a /B or vice versa if one had gone down ? This would help with inventory.
In the screenshot why would the previous engineer use two different ones in the same IO tree ? I can only find manuals on Series B but nothing for series A.

Thanks.

As long as compatabilty is checked under the properties a B should be able to replace an A but I don't think a B can replace a B. I did have a problem with a C version working with a B but I do not believe that compatablity was checked on the controller.

TN49853 - 1794-AENT: What are the Differences between the Series A and Series B? (Access Level: TechConnect)

Speaking about funcionality, they are the same stuff. The technote explains the differences in hardware, firmware and EDS.

Thanks ! Series B is just newer but both are interchangeable according to what I understood. Regardless I'll try to move forward with B.