hum, let me get a stab at it
Well they are indeed made by Valvo in Germany at the Hamburg plant
F X3 it's the type of tube's construction.
G suggests 1957, 9 September. it is strage tough that the G is before the numeral (9), in all the ones i have seen its normally the other way around?????
In that case they should be made in July 1959, I think the latter to be the more likely case
and yes do test them before using them. if they work you will be a very happy man
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