I had the same unit (Shanling) sent to me as a review sample and I replaced the stock tubes with Western Electric 396, supposedly the best version of this tube ever made.
The WE's I received were a matched pair, new in the original box and my results were certainly different than yours. I heard no difference between the super NOS WE's and the stock tubes.
To directly answer your question, I don't think there is a problem with the tubes being marked differently, the 2C51 is a sub number for the 396 according to my tube substitution handbook and not at all that uncommon to find them marked both ways.
Good luck with the tweaking.
The WE's I received were a matched pair, new in the original box and my results were certainly different than yours. I heard no difference between the super NOS WE's and the stock tubes.
To directly answer your question, I don't think there is a problem with the tubes being marked differently, the 2C51 is a sub number for the 396 according to my tube substitution handbook and not at all that uncommon to find them marked both ways.
Good luck with the tweaking.