140,000 Royal Naval reserve records (BT377/7) covering period 1908-55, are now online on the National Archives Kew (TNA) website. I think it was done very recently with no press release.
I think following is a match
James Alexander MILNE, born Edinburgh, 1 June 1881, Number given SA310
It appears that he has added two years to his age, which was very common.
SA indicates that he was second hand on a trawler which is equivalent to a mate.
You can download a copy for about £3.50 but note that the original document was around A3 size,which TNA have then copied as a microfiche. The downloaded copy will be A4 which can make it difficult to read. The original card should be held at the Fleet Air Arm museum at Yeovilton and they can produce an A3 copy which is more expensive but much easier to read. They will also give you a price quote before you order.
I hope this helps you
best wishes
ernestjames