Can the opinion letter date be entered on lin 22b of the 5500. I heard that a plan is more susceptible to audit id line 22c is no and 22b is blank?

Since the instructions don't make it clear, err on your side. I put the Opinion Letter Date in there for a few years, then figured that the Code on the front page indicating a Master/Prototype or Regional Prototype was enough. I agree, though, that it looks awfully bare in that section by answering " " and no.