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• #26
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• #28
never seen a holes in the lug itself seen them hundreds of times on the tubing though, for which i suspect as you said is for draining any moisture, having said that any moister inside wont escape from that hole as its near the top, so i couldnt see much harm from filling it in.
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• #29
The hole is to release hot air during welding, nothing to do with draining moisture. It'll be fine to fill it.