[claude.france]

really , indeed , no , not to plug a hole . but as a lamp replacement

I checked under the chassis .

in fact the 56 has been set there, I suppose ,  as replacement of the right lamp , but, we can say ,  correctly .

You can see on the followig photo, one of  the  6.3 V ring wire is not directly connected to the lamp socket , but via a small wired resistor on a steatite rod, so  placed there to absorb the 3.8 exceeded voltage .
you can see this resistor  beetwen the  two white " tassels " on the photos )  

It appeared that tis resistor is cut , not burned it seems , but just brocken .

If the radio was used innthat configuration , the limited consumption of this fitting limited the current of the 56 heating at normal valu , so 1 Amp , so only 700 mA more could be with the rignt lamp ,  instead of 2.5amp if the 56 were connected directly to the 6.3 Volt , so possibly not enough to overload the 6.3 V transformer output .

Perhaps also the 56 were under heated , using a resistor of more value il the calculation was made to maintain a 0.3 Amp consumption