Difference among Mullard one Getter & two Getters?

Hello Folks!

I received a lot of used Philips branded tubes that seems me was made in England by Mullard, as all these big quantity, a 50 tubes, all have the "Hole" in the guide plastic base pin...+ from some I can read; others no... B on the lower part + other combinated letters & numbers and in the top mixed XF2 > XF3 > XF4.

Will make me very happy to know exactly the differences among One getter or Two geters..., that I can see it inside the tube at the top!

Thanks in advance for your lovely response!

ALEX