Brit. 303 Question, What Do I Have?

I have a .303 that has Crown Stamps with GRI under the crown all over it. On the right side under the Bolt (bolt action) and above the trigger on the stock plate it is marked, GRI ISHAPORE (w/Crown above it) 1939 SHTLE III . What do I have and where can I get parts for it. I have a Lee Enfield No4 MKI which looks nothing like this one. I just need the Safety Arm in order to put in Safe or Fire position. Thanks for the help.