MarceloVeneziale wrote on Apr 25^th, 2012 at 10:03am:

CS Team: I have discovered a problem to place the registration at the bottom of the left wing, the same does not exist. In the file mentioned cs737_03_T.dds see the wing, but I see so much to the left and right. Can be corrected? Best regards, Marcelo

We did it on purpose for the optimization sake. One of 10 (or 100) aircraft has the registration on the bottom side of the wing, so we've saved a whole texture sheet which saved (and will save) a lot of users' video resources and gigabytes of traffic.
Nevertheless you can put the registration on the side of the wing.