I have the same experience, and had to resort to using magnifiers to try and determine if there was a little more of a chip where the directional 'tooth' had snapped off. Fortunately, there was just a tiny extra piece missing where the tooth protruded. Once found, it is a great idea to mark with a black dot on the tube base, just where this tooth pointed.
Broken plastic guide on tubes - how to tell which way the tube fits in socket?
I have a few tubes (and also a set of 6SN7>6F8G tube converters) where the plastic "guide" with the notch that tells you how the tube fits in the socket is broken off. Any way to tell which way the tube or adapter is supposed to fit in the socket without experimenting and frying the tube?