The bung is soldered in, not welded, and there is no flaw- these things are 40+ years old; you can't expect things to last forever. This is probably one of three things:
-The solder joint for the bung cracked and is leaking
-The PO left off some of the padding and the mount bracket wore it thin, which then cracked
-Water in the gas over time rusted pinholes in the tank that may be invisible (or nearly so) to the naked eye
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