Dear Mike
Thank you for your enquiry. To answer your questions, LMtrMch stands for Leading Motor Mechanic, and the service number P/MX99486 comprises 'P' for Portsmouth as his assigned Port Division, which was one of three — the others being 'D' for Devonport at Plymouth, and 'C' for Chatham. The 'M' stands for Mechanics Branch, and the 'X' is for Hostilities Only personnel, as distinct from prewar Regular Navy. His award as notified in the London Gazette only holds basic information which reads:
For outstanding courage and skill in action with the enemy in Light Coastal Craft
To get more specific information you would need to obtain the file containing the original recommendations made by the boat's commanding officer, if they exist in the archive at Kew, as some do. In the crew photo of MTB 606, the Petty Officers wear jackets similar to those of the officers, as well as a peaked cap, though the cap badge is smaller. In addition Officers display rings on their sleeves, one for a Sub-Lieutenant, two for a Lieutenant, and one broad ring with a ring top and bottom for a Lieutenant Commander, usually Senior Officer of Flotilla. Leading Seamen would have worn the same as sailors, with a small insignia on their arm, whilst a Mechanic or a Stoker (Mechanic's assistant) might be dressed in overalls as workwear. In the photo of the crew with the flags, the Mechanic may be the one in overalls holding up the item of clothing, but any member of a boat's crew could potentially wear overalls.
If the family no longer has the citation which would have come with his award, or his full service record given him on his discharge, then more information might be gleaned from obtaining his service details from the Royal Naval Disclosure Cell. If you have his CFVA membership number then it might be possible to find further details of his service if he provided them to the CFVA when he joined. Alternatively if I know his date of death, which may have been noted in a newsletter at the time, I may be able to determine his CFVA number that way.
Regards
Admin