1. The brass and aluminum models were made made "back-to-back", within a relatively short period of time, at the cusp of the transition from brass to aluminum casting. The lack in details in both design and finish may then have been a result of oversight in quality control due to more important priorities relating to the changeover of manufacturing processes,
or...
2. The reversed swastika was intentional for whatever reason(s),
or...
3. nobody really cared as much as we would like to believe they cared,
or ...
4. Simply, nobody noticed.
As you say, whatever the reason, we will probably never know it (... unless somebody can come up with a period Voos catalog that pictures this "variation" )
)