551 vs 550
Although I stand to be corrected, it is my understanding that the 551 is the same aircraft as the 550 and the 501 the same as the 500. The only difference is that your license is for single pilot operations 501/551 vs multipilot operations 500/550. Your LPC will determine which rating you have on the same aircraft. You must be tested in the role which you operate. ie single pilot LPC= 501/551 rating. Multi crew LPC=500/550. You must have a valid LPC for the role you operate. You can not operate 501/551 on an AOC as all turbine night or IFR public transport flights must be multi crew. If you have a 551 rating you must revalidate for a 550 rating.
MM