Step 1. Failed. The optical drives are not swapable. Drive A on the left is from the 400Mhz unit, Drive B on the right is the 667's. Not only are they different physical sizes, but the connectors are on opposite sides. The 400 can go back together and back on the shelf now. Maybe I can use the 667's drive in the Pismo? I'll try that after.
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EDIT: A little off topic, but the Combo drive works perfectly in the G3 Pismo. This is the first time I've had a chance to see DVD Player in action with the 8MB Rage Mobility 128 and it is surprisingly fluid on this old Mac.
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The fit is flush, but not perfect (as it's a slot loading drive in place of a tray loader). It would be nice to cover the gap below the slot, but I'll live with this for now.. Maybe I could cut out the bottom half of the original tray bezel and glue it on underneath the slot bezel. Disc Burning is recognized as expected.