1946 Piper PA-12 Super Cruiser Fuel Tank's

Hello
I am a welder by trade , used to work as a certified A/C welder for major
company , still do Alum . and 4130 A/C welding , a man brought a wing tank
to me from his Piper PA- 12 Super Cruiser today , he has a small leak around
one of the fittings , the tank has been soldered together , he says it's
Alum. but I am not convinced , the sheet metal is very thin and is non
magnetic , I scraped some of the solder material and it was soft like lead ,
the man called an older fellow A/C restorer and he thinks it's Alum , all
the fuel tanks that I have worked on were made from 5052 of whatever
Alum.and TIG welded , 1946 may pre date the Heli-Arc process , wonder if
someone can shed some light on this wing tank question.
Thanks
Phil
EAA 12873