The GI-21 is the larger counterpart to the GI-10, both manufactured by National Union as part of their “Inditron” line of display tubes. This places it among the earliest Nixie-style tubes ever produced, even predating the term “Nixie” itself that was coined by Haydu Brothers (later Burroughs) in the late 1950s.

Like the GI-10, the GI-21 does not feature a dedicated anode cage, which became standard in later Nixie tubes. Instead, to illuminate a specific digit, all other digits must be held at anode potential while the desired digit is connected to ground. Interestingly, the GI-21 includes four metal rods connected to one of the pins on its phenolic base. In some examples, such as the one shown below, these rods can function as a dedicated anode, significantly simplifying the required driving circuitry. However, this functionality appears inconsistent across different units, suggesting that it may have been an experimental or partial implementation of a dedicated anode that did not fully meet expectations.

The National Union GI-10 is likely the first Nixie tube ever produced, belonging to National Union’s Inditron series of display tubes. Patented applied for in 1954 (and granted in 1956), it predates the original “NIXI” tube developed by Haydu Brothers and later Burroughs by at least a few months. Similar to other early Nixie tubes, such as the STC GN-1, its digits are not stamped from sheet metal but are crafted from wire. These digits are connected to the tube’s 10 pins via long rods covered in an insulating layer of ceramic that also serve to hold them in place. In contrast to more modern Nixie tubes, where the digits are arranged to minimize obstruction of each other, the digits in the GI-10 are organized in a straightforward, sequential manner. The zero digit is positioned at the very front, while the one is located at the farthest point in the stack. Unlike most later Nixie designs, the GI-10 lacks a dedicated anode; instead, activating a specific digit requires all other electrodes to be held at anode potential, which complicates the driving circuitry and makes it difficult to achieve uniform brightness across all digits. The tube uses a standard Noval 9-pin socket with an additional central pin.