I was given the prefix "PO" to my service number, as I was in Portsmouth Company when I arrived at Lympstone and so was the rest of my troop. There were 2 back troopers at lympstone and joined our troop who had the prefix "DE" to their service number. I believe they were in "Deal" company when they arrived. I think the third company is "Plymouth" company and they rotate new troops between themselves as they arrive. Hence why you'll have in training a "Troop Commander" and a "Company Commander" above him.
I think that's how it goes in the Marines, as when I told it to someone from the Army. They thought I was an officer as he told me that when someones has letters before their service number that indicates they joined as an officer.
It's not 100% sure, but I think it follows along those lines
Sorry buddy, your worded it right mate, it was me being up early on and not concentrating
back onto your question now I have read it with my eyes open. It's basically just a sequencial number for your troop. It's been going on like this for the last 25 years or so. Every troop that joins goes up by one and it's just to identify your troop when allocating booking times for the gym and other facilities.
It was something similar to this before it as well but got reset in the early 80's I think.
You will get dog-tags sooner or later, but I wouldn't be too concerned about them. You normally get red-fibre circular ones for peacetime, which you keep on your respirator (gas mask) haversack & metal engraved circular ones when you go operational. (Not like the rectangular US ones). You get two tags, name, rank service number, blood group. One on a large chain to go around your neck, the other one interlinked on a little chain, just big enough to go on your big toe if your head falls off.
(well, that's how it was explained to me when we were issued the stainless steel ones!)