Actually you get both the turmumbau II and III's too early in GWX. According to uboat.net the turmumbau II should be available in November 1942 (not January). The turmumbau III should probably become available in the later part of 43.
I just edit the basic.cfg to make it more historically correct, but perhaps this "issue" is beeing overlooked in the upcoming 1.04 release.
Btw, here's an older thread regarding the dates for the tower upgrades.
http://www.subsim.com/radioroom/showthread.php?t=87999