Your rifle is an M.84 which was a dedicated single shot target rifle.
While it isn't a prototype "C3", it does belong to the M84/M86/M87/C3A1family.
Someone has removed the safety lever from the trigger.
Interesting that the J is a prefix, and that the top rail of the receiver is NOT marked with Parker Hale M84.
Your rifle may be a Pre Production or prototype M84, "Gil Boyd" might be able to tell us if he had the full serial number.
Attachment 87296Attachment 87297
here is an add for the M87 (the 'doner' C3A1 action) & M84 and a pic of my M84, built into a magazine fed rifle by our own RWSGunsmithing
As a side note:
The M85 action, while fitting into the model sequence (and definitely related) is more of a parallel development.