I lined them up face-to-face and inserted a stick between them as a reference.
If it were electrostatic focus, would there be more than 5 pins on the base ?
Here's a side view with the 17BP4 on the left and unknown on the right. The curvature is less, but I wouldn't call it cylindrical.
Here's a top view with the 17BP4 on the top and unknown on the bottom.
