but regarding the rarity of the 7th versus the others..
I think it might have to do with it being "unfinished".
The enameling was to be either blue or white,
but something happened and it wasnt decided..or they didnt want to pay the cost...not sure..but maybe for this reason there are less badges issued of this unit than the others..
It certainly seems there are less of these out there than the 9th and it seems as well as the 8th