Can anyone reccomend a UK supplier of good quality 1/4" (or 6mm) brass button oilers, just had some delivered from a "northern supplier" and not one has a circular hole where the ball sits all have some form of ovality, lip or other deformity rendering them useless.
Paul, you might be able to improve those you have by upending them over something with a hole in it to accommodate the projecting part of the ball, then using a thin drift and hammer to give the ball a light tap or two on the back. That might make the brass lip of the hole conform with the ball.