[old_tv_nut]

Under the principle of "fix the obvious first," the straight edged picture is with one yoke and the curved edge pic is with a different yoke, right?

I would suggest going back to the first yoke to verify that this change is due to the yoke and nothing else. If that's the case, it may be that the yoke that produces curved sides has either a different winding pattern and or metal/magnetic elements that produce the curve and is intended for a different CRT/receiver, on which it would produce straight edges. Or it may be missing some elements that would correct the curved edges.

Can you see any adjustable pieces on either yoke?