Electrically similar beam tetrode with same pinout but different internal construction and slightly different characteristics — often interchangeable but not identical
Triode characteristics from GEC datasheet page 2 at Va = Vg2 = 250 V: gm = 12 mA/V, ra = 670 Ω, μ = 8. Schlangen measurements give lower gm (6.0 mA/V) and higher ra (1.3 kΩ) — likely at a different, lower-current operating point.