I'm not entirely sure about the square or round. Maybe your manual will have a small diagram. First thing i'd try to do is get some electrical contact cleaner and spray it into the key fob(battery disconnected. Put your key in and out, turning to each position until they key is dry. If this doesnt work, disassemble the front gauge cluster area and on the back of the ignition is where all the electronic input comes in. Disconnect it there and spray some more cleaner, let it dry and check it again.
If this fails, there are several places on the internet that sell new key ignitions(key included). I went ahead and put a new one in my bike, keep your old key for your seat lock, and i havent had any problems like yours since then.
I ordered one with the 5 prong, like seen in the link below. Once you disassemble your cluster, you'll see what type you need.
http://www.partsnmore.com/parts/honda/cb750/?filters%5Bcategory%5D=electrical&filters%5Bfitting%5D=custom
Locked
