It might look like B5 but could be something else, or from a different year. Similar colors were even offered at Maaco & Earl Scheib.
Add to that there are variances within a system, and at least 4 different major paint companies, each with their own internal code for B5.
Your painter should have a jobber that can determine the color with whatever system he's using to help narrow it down.
He or the jobber should also have sample books that might have something close enough to be tintable or blendable if you're using a basecoat/clearcoat system.