The system is fully expandable to 65K, with the entire data and address busses, clocks, control signals ( i. e. IRQ, NMI, DMA, RDY, etc.), and power sources available at the connector.
Were any of those (few) sold boards ever expanded to the maximum possible RAM? That would be a beast of a machine, for cheap, in 1976.
64K of random access memory, plus 1K of linear access (shift register) memory.
The 1K shift register was used as the 40x24 screen buffer. This is the biggest limitation of that machine; you can't modify the screen directly, you can only push characters on to the end like a serial terminal.
I think what would surprise younger people is how in the very old days it was unclear that RAM would win over all other forms of access and there was quite a bit of non-random access hardware available such as "bubble memory" and CCDs and various delay line schemes and magnetic drums etc. Much like trying to tell young people that tape was once used as primary storage for computing not as backup-only storage.
On one hand its annoying having to pump addresses and later rows and columns at memory vs "here's a stream, best of luck using it" but the program complexity and performance gains of random access are very high so RAM wiped out ideas like using CCDs as anything other than video sensors.
Seeing as tech is an endless wheel of everything old is new again (see 70s programing paradigms and stuff like that) "someday" someone will try to disrupt some corner of computing with non random access memory.
The experience would likely be similar to ladder logic programming where all the instructions operate repeatedly and simultaneously, well, at least as a programming model. Another analogy would be spinning rust hard disks, here's a continuous stream of bits pouring off the drive head, now write a program to simulate a block store that uses that bitstream, and that is called hard drive firmware or back in the old days that was a MFM or RLL drive controller.
Were any of those (few) sold boards ever expanded to the maximum possible RAM? That would be a beast of a machine, for cheap, in 1976.
Also: 65K of memory? Not 64K?