I don't have any lying around, it would take about 4 to 6 weeks, most likely.
You're actually referring to two different barrels here, though. The 24 Special barrel is a standard bull barrel (fluted), which uses a standard gas tube. The one Midway and ourselves refer to as having the modified gas tube is the _Super Bull_ barrel. I don't sell them without the gas tube, so the ones Midway is selling will have the gas tube, IF it's a super bull.
The average wait on rifle orders is about 4 to 6 weeks. Incidentally, I have 1 (one, uno) 24 Special that just came back from a show as a display model. I would guess it will be gone in short order, so have your dealer call ASAP if you want it.