Not everything has to be NOS or new, how else are you going to ride a bike from a certain time period with a certain component group without worrying about scratches?
RD-M910 is the model name of the top end Shimano rear derailleur of the early 90’ies, the XTR rear derailleur. However this is not the first XTR rear deraillur but another revision that had a much softer spring in it. Shimano meant it would make the action a bit nicer and I do not think I have to disagree about that since it was a nice combination with XTR shift/brake levers.
The XTR rear derailleur is very nice looking in anodized aluminium and nice soft silver/blue look. The XTR logo is clearly visible.
This fine derailleur in good mechanical condition would look great on any vintage mountain bike meant to be used, it is yours for 50 euro.