This is his service number. It is made up of
P = shows that Portsmouth was his Port Division, for welfare, drafting etc
SS - Seaman or Stoker
X = on the post-1925 pay scale
832607 = service number
AB - Able Seaman
There is no significance in a missing initial - it is not unknown. It probably means that when the forms were filled in, your fathers name was one in a list, and the person completing the details accidentally missed out the first letter