17 May 43 , U 128 a IXC
one 37-mm. (1.5-inch) anti-aircraft gun abaft the conning tower, one 20-mm. anti-aircraft cannon on after part of the bridge, four demountable C-38 machine guns on bridge, one on each end and side.
Sounds like an IX/1
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31 July 43, U199, an IXD2
One 37-mm. Aft. One 20-mm. On Platform
http://www.uboatarchive.net/U-199INTPlates.htm
Definatly an IX/1 (this suprises me)
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14 Aug 43, U185, IXC/40
One 20-mm. Anti-aircraft gun on Platform I and one on Platform II.
Definatly an IX/2
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5 nov 43, U848, an IXD2
Looks like an IX/3 tower:
http://www.uboatarchive.net/U-848.htm
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13 Dec 43, U 172, an IXC
1 – quadruple mount AA gun on Platform II, fitted with two armored shields.
2 – twin mount AA guns on Platform I.
http://www.uboatarchive.net/U-172PlateA.htm
Definatly an IX/3
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I guess from this we can assume that the IX/3 tower didnt appear until the later end of 43.