In early September 1914, for the first time in 250 years of RM history, short service recruitment began. The RMLI Divisions Plymouth (Ply), Portsmouth (Po) & Chatham (Ch), all opened enlistment for three years/duration of war (also the RMA) – THIS IS WHERE THE 'PO' COMES FROM -Portsmouth Division.
For your additional information Long-service enlistment continued, but with a wider acceptance of candidates than pre-war preferences would have allowed; that is to say, almost anybody would now be accepted, due to the urgency of the required numbers & competition between the services for volunteers. Before the war, only men of proven good character were enlisted. A man/youth wishing to enlist for long-service would have to wait while letters & enquiries were sent from the RM Recruiting Officer to the local Police & former employers to affirm his good character. Only on receipt of these references could his enlistment proceed. This practice continued until November 1914 but would then appear to have ceased. Recruits were so badly needed that any size or shape could now join the Corps. This new policy was really put into action with the transfer of 600 men of "Kitchener's Army" to RMLI short-service. Training for the short-service RMLI differed greatly from their long service counterparts.
Short-servicemen were trained for just over six weeks in infantry skills only, before joining their respective battalions in the RM Bde. They received no examination certificates for their attainment of the required standards in Musketry or any other drills. Their training was undertaken at the Division's home barracks, unlike long-servicemen, who were sent to the Recruit Depot at Deal on enlistment, where they were trained & tested in Naval Gunnery, Musketry, School Certificates (3rd, 2nd or 1st Class) & swimming. The results of all these examinations were entered on their service sheets & certificates issued for some. Long-service training took nearly six months to complete, before they were dispatched to their Parent Division's barracks to await their postings.
Before the war, long-servicemen were trained at Deal for a whole year, so their six months of training was actually a shortened course, to facilitate their early disposal for active service. At the end of 1914 the Royal Marines had over 10000 men serving in the Fleet, with a further 4500 in the RM Bde. & many others serving in numerous foreign stations or home base commitments. Almost all the short-service RMLI were committed to the ranks of the RM Bde, making up about a quarter of their numbers, the rest being long-service RMLI.