Photo-multiplier Tube Pin Configuration
GS Tube doesn't make the FEU49B, or any other tube.  They sell new old stock Soviet produced vacuum tubes.
The datasheets are therefore difficult to read - I know English and German, but no Russian.
As I understand your question, you are looking for the pins for &quot;Photo cathode&quot; and &quot;Modulator,&quot; and can't find them on the pins that go into the socket.
They aren't there.
They are on the tube itself.

That picture from the GS Tubes FEU49B page shows the K (cathode) to the far left of the schematic representation of the tube.  The drawing of the tube to the right shows the K connection on the side of the tube.  (At the bottom of the drawing of the tube.)
Your other missing pin is then the one marked &quot;Y9&quot; (the concentric circles) drawn on the tube.