Are you simply looking to identify the external appearance differences, or more detailed body fittings?
If it's the former, the following are the main visual differences: P5B will have built in fog lights in front wings (but some P5s have been fitted with them) Chrome strip is thicker than 3-litre coupés, (other than the MK3, whose strip ends in 3 chrome pips at the rear) P5Bs should have chrome strip on cills under doors Black painted lower cills Different radiator grille, brassy coloured badge Rubber faced over-riders, flatter faced. Twin outlet exhaust Twin orange repeaters each side (1 front, 1 rear) Different style badges No hole for starting handle on front bumper Ro-style wheels. I've seen a few, but not many, P5s with Ro-styles, but never seen a P5B with the older steel wheels/trims Coachlines The colour options are gdenerally different, although some later MK3 P5s had a similar dark blue colour
The above are the obvious ones I can think of off the top of my head. Numerous interior differences, but you haven't asked about those.
I know that MK3 fuel fillers were chromed and different to earlier 3-litre ones and then no doubt carried on to the P5B unchanged.
I couldn't remember, so I checked and according to James Taylor's book the filler cap was changed from chrome to body coloured flap in early '63, which is odd really, considering the P6 was introduced that year with chrome filler cap and they changed the P5 back to chrome for the MK3.