shimano chainring compatability

waterford45

I have a shimano 105 (current model) triple chainset with a 50t outer ring. Looking to replace the outer ring. LBS gave me a dura -ace 7800 50t as a replacement at a very good price. it is still in a sealed bag. one of the chainrings is stamped with a 'b' and the other with a 'd'. my reading of shimano tech diagrams suggest that they may not be compatible, but lbs said there would be no problem. bolt circle diametres (if this is the correct terminology) is the same for both chainrings. any advice would be appreciated.