Hi Stuart,
The "safest" way to put it on free play would be to remove the "hot" side of the keyboard button release switch.
This will cause the latch bar solenoid to not activate, forcing you to make all selections by pressing and holding a number, and letter key down at the same time.
Do you have the service manual PACKAGE, for this phono ? If so, read, it, the "I & O Manual", and the "Trouble Shooting Guide". If you do not have a "general knowledge" of the Seeburg mechanism, I would humbly suggest a copy of my "Seeburg Mechanism Guide", would help, a great deal. -I sell this directly, just PM me, if you wish to purchase a copy-- Ron Rich