You wrote
"He was employed as a Fireman, which I believe means he was what we now refer to as a Stoker, on a wage of £4.10.0d per month. As he was away for more than 5 months I am puzzled about his final wage - £12.9.6d"
It could be down to several cause's but the most likely is he drew a "Sub" to be used for a run/s ashore. He purchased essential items of clothing and other gear from the ships store. He made an allotment to be paid to a person/s onshore usually a family member, wife, mother etc.
It is always "Fireman" in MN parlance. Stoker was a RN rank.