The original Atari 800 could have up to 48K of RAM installed through the use of memory cards. When the 800XL came out, it was now possible to have up to 64K of RAM by disabling the BASIC ROM.
After reading this response about bank switching in the Atari 8-bit era, it led me to dig a bit and realize that the 800XL bank switched the BASIC ROM in or out of the "left and right cartridge" space of RAM (each "side" was 8K thus 16K total). On the original Atari 400/800, BASIC was not a built in ROM but rather a (left) cartridge you would put in. So when you booted an 800XL with BASIC disabled, it was equivalent to booting a 400/800 without the BASIC cartridge.
So given that you could boot an 800 without the BASIC cartridge, I'm wondering if there was ever a RAM card (either expansion under the hood or as a "left and right cartridge") that allowed you to have that extra usable 16K in an Atari 800 when not using BASIC?
I'm guessing it would come down to either the 800's RAM card cage not having enough lines or the cartridge ports not having a R/W signal, but either way - it something I'd never thought of in the last 40 years :-)