SuperAMS

"The SuperAMS circuit card for the P-Box replaces the 32K card and allows as little as 128K and as much as 1MB of memory for use by the 9900 processor. This is accomplished by remapping the eight 4K segments of the 32K expansion memory space under the control of the Texas Instruments 74LS612 memory mapper device. This is the same basic technique used in the Myarc 9640 computer and the never-released 99/8 computer."

Thanks to Carlos Randolph for sending me the picture of his card which he built!