NO. 3 Commando

Hi i am looking for some information regarding my grandfather during WW2. i know he was in the marines i just don't know which unit he was in. in looking into where the NO3 commando was during the war i think he could have been in it but unsure. i found a few of his things and took some photographs but i cannot get them uploaded. his division and register number is PO/X114212 Not sure what it means if anyone can help please do