Before we embark on a lengthy discussion about the aggressiveness/mildness of the non-adjustable Red Tip Superspeed in combination with various razor blades, did your uncle use it recently before the razor came into your possession?
It is possible that the butterfly mechanism is worn out and the razor could be out of tolerance.
I also don’t know what you were trying to adjust “at zero (longest cut)” as there is no adjustment on the Superspeed. You twist the base solely to secure the butterfly doors, but that needs to be done properly (sufficient, but not too much force) to secure the blade in the razor.
B.