Hi,
The static transconductance test is more commonly known as the
grid-shift method. It's very old, dating from the 1920s, but can be
extremely accurate, depending on the amount of grid shift (easy with
modern digital meters). Grid-shift went out of favor in the US in the
1930s but most of the British AVO models use it, except with rectified
AC on the plate instead of DC.

The Hickok AC-47 from 1930 had a dynamometer movement (6.25mA F.S.)
and is actually a very fine instrument but was dropped in the mid-30s
because of expense and the difficulty of keeping it up to date with
adapters. Other than one Sensitive Research design, the last
dynamometer model was Hickok's "laboratory model" 700. That beast was
obsolete the day it appeared in 1951.

The Hickok Cardmatic models---123, military USM-118B, Western
Electric KS 15874---can measure either emission or transconductance,
sometimes both (on TV sweep tubes for instance where extra cards are
provided for each test).

Alan