Interesting that it is a linear moving coil ammeter and only marked up to 100mA, moving coil meters are DC – the ones I’ve seen on the 1154 and 1155 set-ups are usually ‘Hot Wire’ types used to measure RF current in a direct manner and usually much higher current, in fact 4A is shown in the previous drawing. Being ‘hot-wire’ they had non-linear scaling (again see previous drawing).

It is in exactly the same sort of case as the 1154 and 1155 with an in-circuit and out of circuit type switch (i.e. don’t waste precious antenna power showing me the antenna current!)

It’s possible to have a linear meter for the purpose but with different internal circuitry, this one seems to be an exception to the general rule – worth looking into more perhaps, rarer, later?

Cheers
James