Before most of the code in the designer was using EF6 but validation still used EF5. In EF6 we introduced new manifest tokens for Sql Server (one for 2012 and one for Azure) and the code that was still using EF5 would fail on new manifest tokens. To workaround this for now we added the PreviewHelper class that was rewriting newly added manifest tokens to 2008. Once we move validation to EF6 the class must be removed and we should use the new manifest token for EF6 models.
Note that for EF5 models we must not use new manifest tokens.
Comments: I tested this on EF5 and EF6 with localdb and SQL Azure. The right manifest token was generated for the SQL type and EF version. EF6 on SQL = 2012, on SQL Azure = 2012.Azure EF5 on SQL = 2008, on SQL Azure = 2008
Note that for EF5 models we must not use new manifest tokens.
Comments: I tested this on EF5 and EF6 with localdb and SQL Azure. The right manifest token was generated for the SQL type and EF version. EF6 on SQL = 2012, on SQL Azure = 2012.Azure EF5 on SQL = 2008, on SQL Azure = 2008