The RCA Selectron -- 1024-bit Selectron prototype No. 4124-3
The model 4124-3 Selectron is organized as a 2 by 512-bit memory. The glass envelope is 75 mm in diameter by 200 mm tall, with the evacuation tubulation at the top. Initial fabrication was Summer of 1948
The internal structure of the planar design Selectron is more complex than that of the 4096-bit cylindrical design. Instead of storing charge on a flat surface storage occurs on electrically floating discrete metal eyelets. 1024 eyelets (512 each face) are fastened to a mica sheet, which is sandwiched between several other active layers.
Hundreds of spot welds connect thin nickel ribbons to the internal elements and to the 48 electrical connections on the base. The base and electrical connections has advanced from that of the 4096-bit prototype and uses the fritted glass "button" base similar to that used in 7- and 9-pin miniature receiving tubes.
There are actually 50 pins, but two are trimmed to be used as a locating index for orientation of the base allowing proper insertion into a socket.