Unlike modern RAM upgrades, this one is an actual vintage wire-wrapped 48K upgrade constructed in 1981 or so.
Atari 400 homebrew RAM expansion obverse. Note the wirewrap connections and pin 4 has been cut. Click image for larger view.
The wire jumpers added to the motherboard to allow for the RAM expansion to be made available to the system. Click image for larger view.
Here is the RAM expansion as it would appear installed within the Faraday cage (removed) within the Atair 400. Click image for larger view.
So how much RAM is available? Click image for larger view.
When I test the RAM in basic using the command ?FRE(0) the system reports 29690. In theory this RAM card has chips enough for 64K. I tracked down the original notes and schematic for this RAM daughterboard to see what was going on.
Apparently the board was originally wired for 48K. In 1984 pin 4 was cut to step down the board to supply RAM from 00-32K only. With the advent of the 16K game cartridge (such as Pole Position) RAM space 32K-48K was needed for the larger carts. Essentially without a switch (electronic or software) RAM expansion for a 400 maxes out at 32K, if you want to use 16K carts.