the old frame could have been damaged ,and the owner bought the best frame for the money at the time
it doesnt detract from the bike in my eyes but some people it will !
if your well into it and its an investment id be on the lookout for an mc21 frame ,just to make it right
no one would ever know anyway except an expert on nsr's
once you have got a frame sell your 28 frame and get your money back ,
good luck with it, if its in good condition change it if not ? its your choice
also a bartering point with the seller !
good luck with it mate !
still not ridden mine yet ,