[MasterGunner01]

No, both the ONTOS and the SPAT did not use the same chassis. They both had their engines in the front, but the ONTOS driver sat next to the engine. The SPAT driver sat behind the gun shield for the 90mm gun. The gunner, commander, loader, and driver we completely exposed on the M56 SPAT. The driver, loader, and commander were carried inside the M50/M50A1 ONTOS. Both SPAT and ONTOS carried ready rounds beneath the fighting compartment floor; ONTOS had an armored cover over its ready rounds. The ONTOS had to pull out of action so the loader could reload the six external 106mm recoilless rifles from the ready racks. ONTOS carried 12 rounds; six in the rifles and six in the ready racks. The M56 SPAT carried 29 ready rounds.

[Rockpile]

Well, now I had to actually look up the two vehicles. The Ontos prototypes at first had the running gear of the M56. Reckon that is what I dredged out of the memory mud.

The Ontos was a cramped sucker inside, too tight for my tastes.
Only M56 I have seen was outside a VFW hall. Gone now.