OT USB to PS/2 adapters

[Mark Jarvis]

I have a relatively new (<1yr) PC without PS/2 keyboard & mouse 
connectors. I've accumulated 4 USB to PS/2 adapters. Three are just 6" 
cables with a USB male on one end and a PS/2 female on the other. The 
fourth is similar, but has a large oval-ish thing in the middle of the 
cable. My MS ergonomic keyboard has a PS/2 connector and will not work 
when connected using one of the three plain adapters, but will with the 
fourth.

I'd like to understand a) why, and b) where to buy a couple more of the 
adapters that do work (for another project).

Any help from the incredibly knowledgeable PLUG-ers will be gratefully 
appreciated.

Thanks in advance,
Mark Jarvis