The prohibitory sign is caused by macOS detecting you have an invalid model (one that is not in /System/Library/CoreServices/PlatformSupport.plist). The methods around that are to skip model verification using a kernel flag, or adding the model to the file (which can get overwritten with updates as seen in your situation).
Note that you also add any kernel extensions and support files if those were overwritten too.
Thx for your help.
However I found a different and very simple route to the solution.
I booted from the partition with OSX El Capitan, mounted the external harddisk with macOS Sierra, and with CarbonCopyCloner I then could repair the macOS Sierra Recovery Partition.
Repairing of the Recovery Partition took less then a minute (6 files where copied) and now the macOS Recovery partition on the internal harddisk is bootable and all tools on it are available.