[turretslug]

It's not uncommon with IO and similar base valves- the leadout wires may have been heated and therefore a little tarnished during the bulb processing and sealing, and the soldering at the end of the hollow pins may have been both cursory and minimalist. I've often found dry-looking joints with just a tiny amount of solder in the end of the hollow pin, and that rectifiers with their high current heaters seem particularly prone to joint iffiness.