I would certainly assume that "BN" refers to common much more frequently than the L182. There are tons of different species of pleco, normally if somebody has gone through the effort to get a rare one they'll make sure to mention it. For instance I would never refer to my L183 (ancistrus dolichopterus) as a BN or my L204 ("flash" panaque sp.) as a clown. If he is talking about BN then the ceiling for growth on them really is 5 inches. But you have a point that without definitive species ID it is hard to say as there are as many sizes of pleco as there are colors.