Thanks for your answer.
Now, let me get this right (I do not want to go through flashing recovery yet again!)you should be able to install stockrestyle also over 3.1 update
however, you can download the 3.0 recovery firmware from Nvidia, extract the vendor image, download stockrestyle system image and flash only this system and vendor images with fastboot.
you can install stockrestyle system and 3.0 recovery vendor images also ove 3.1 stock.
so if, over my current 3.1 rec, I flash
first: 3.0 rec vendor.img ("fastboot flash vendor vendor.img")
then SR system.img ("fastboot flash system system.img")
I should be with SR - do I need to "fastboot -w" afterwards?
Thanks a lot