Probably the handlebar is not rounded in the clamp area??
Measure that thing in at least 4 different places and see what you get, it could be ovalized some how. Seen old cinelli's come with 1 mm of difference depending where are you measuring it. Probably is a 26 mm depending where are u measuring.
The stem looks like ok to me, Always you can check using another handlebar just in case but pretty much is just thing of putting the front cover and measuring.