Push-pull pots

Push-pull pots are pots with a built-in switch, generally used in guitars and basses.
These are available eg from Mouser or from potentiometers.com (who can supply with expensive custom pots).

The switch is on the bottom of the pot, and has lugs that I will label A-F :
when you pull the switch the connection changes from B-C to B-A, and E-F to E-D.
Use your meter's continuity setting to confirm your switch works the same way.
The "middle" lugs (B & E) are the constant, the "outside" lugs ( A & C, D & F) are the connections you switch between. (from Deaf-Eddie's page)

Some value seem hard to find for push-pull, a solution is to buy common pots of the same type and to replace the push-pull pot's resistive track.

Push-pull pots are easy to disassemble, but the common pots don't (I had to break the shaft to get the part).

This allows having any value you could want.

No hole
Matrix MKI/MKII layout
Voltage controlled shape and response
Hi/lo range switchable oscillators

Filter 18/24 db
Simple Random Voltage Generator and Sample & Hold

Documents, manuals, dopesheets, links

Filter slew/unslew
Oscillator synchronization
Attack and decay time extension
"Synthi 2" interface