That's junk (oily/greasy water) seeping back out from between the ribs & bbls after you pull them from the water.
The void fills with boiling water and mixes with any old oil, solder flux, ect in there and comes back out the way it got in.
Very tough to get that cleaned out w/o a hole for the water to get in and out fast.
Bet those bbls ring like a church bell too.
A tiny hole in the bottom rib just ahead of the forend lug is a start. Sometimes back further by the flats. That one will get covered by the ejector stems in alot of cases when assembled.
You really need another at the muzzle end to get the water to move out quickly when you pull it out of the tank.
Sometimes the front bead hole will lead to the void. If not, sometimes you can drill a tiny hole through the solder that's in between the muzzles and into the void. Carefull you don't drill into a piece of steel filler in there.
Sometimes I just go ahead and melt the solder at the muzzle and shake it out to clear the end. Then solder it back up when done.
Much better and easier than chasing oily water spots around the tubes every pass and hoping the next coat will cover them, cause it won't.
