Phantastron
The Phantastron is a self-gating linear sawtooth generator circuit based on a pentode/triode Miller integrator.
It is used in the 512 and possibly other instruments.
See description on page 26 of this document:
- John Mulvey, Some Basic Vacuum Tube Circuits used in Tektronix Instruments (Third Revision 1964) / (1960 edition)