The downtubes are all sealed by welding at both ends and if the seat tube is dry as you say then there is very little chance (pun) that the part of the downtube that is inside the bottom of the seattube could have corroded and perforated to allow water to enter. If the water bottle bosses are sealed, then it is possible that a minute amount of brazing flux or melted silver could have gotten underneath the boss and dripped into the tube especially if the hole was a little too big to begin with or if the brazer messed it up the first time, pulled it out and rebrazed it.. The other possibility is that the downtube was not welded correctly to the bottom bracket when the frame was in the tacking jig. Since part of the downtube is covered by the seattube (where it is notched), that part has to be welded before the seattube is tacked into place. Since I did not tack weld all of the frames, I wouldn't know if that happened. Extremely unlikely though, even with the smoky shop thing going on
, the tackers and frame fitters were very conscious of this.
The most likely scenario is that some debris got in through the bottle boss hole sometime between when the frame was welded and when the bottle bosses were brazed in and nobody noticed it. Often, the frames would sit on a rack for a week or more between welding and brazing depending on production needs at the time. I know the guys would usually blow them out but probably not 100% of the time.
I would say the likelihood of the substance being water or other liquid is very slim (another pun)
Good Luck
Scott
