I'm currently in a IXB in 2nd Flotilla out of Lorient. I upgraded from my VIIB in late November 1940. At that time, only the IXB was available as an upgrade. As it turned out, by the time my new IXB was ready to leave port (after I tricked it out with several of the upgrades available), it was early January 1941. It was late so I exited the game at that point.
When I loaded back in, imagine my surprise when I went in to get my torpedo loadout set before leaving port and was presented with the IXC as a possible upgrade. Of course, having just blown all my renown on my IXB and its upgrades, I didn't have the renown left to choose the IXC at that point, but it was showing as available as early as January 1941. According to U-boat.net at least some of the IXCs were commissioned as early as January 1941, though I don't know if they were actually available to 2nd Flotilla kaleuns for combat operations that early.
Oh, and BTW - with SH3 Commander you can manually choose whatever grid square you want for your next patrol zone - unfortunately I don't think SH3 Commander has a map showing the grid squares, so you'll need to know what you want ahead of time in order to take advantage of this feature (and of course you'll need to do this when you're in port between missions - otherwise it's grayed out in SH3 Commander 'cause you can't change it mid-patrol).
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