Thanks for that ycaso,
His letter prefixes:
P = Portsmouth his “Home Port”;
J = Covers the Seamen & Communications branches;
X indicates he’s on post 1925 pay scales, the lack of SS means he’s a long service rating, enlisted for 12yrs with option for further 10 for full pension.
Together the letters and number indicate he enlisted between 1934 when the “Home Port” letters were added & 31 March 1943, after which new entrants were given numbers starting from 500000.
The LS&GC Medal was for 15yrs service with conduct rated “good or above”, so assuming his “crimes went undetected”, his medal being claimed in 1954, means the latest he could have joined was 1939, or earlier if he had a Good Conduct Badge deprived then restored.
That could fit with the man in the pictures.
It all depends on when he married, as on a duplicate post Deborah has stated she thinks he was a bus driver at the time and there were a few births of boys named Roy Rogers between her granddad’s in 1910 (?) and 1921 that would make someone 18 to fit with the youngest age to qualify for the LS&CG medal in 1954.
In 1954 HMS Vernon was the RN’s Torpedo Training Establishment.
Are you sure about you grandfather’s birth being in Yorkshire 1910 as I can’t find a registration for him; do have a copy of his birth certificate or have you located him in the 1911 census?