Hi, yes you have got the wrong cog on the end of the left hand side of the countershaft. The shift fork should sit in a guide which is part of the cog, like the other countershaft shift fork does, so the gear can be smoothly moved to be engaged or disengaged from the adjacent one.
If you go to the cmsnl website you can view the microfiche there for your model and this may help you. It is item 13 on the CB750 K3 transmission fiche, yours will be similar. The particular gear just fits through the bearing holder hole.
Good luck with it.