Going by the prefix system.... in the 70s they tended to follow the 2nd letter of the model series for the first letter (ie, HQ got Q, LH got H etc).
Obviously we know there were no J or X prefixes, but Z came in during HZ.

Into the 80s, they used the platform code for the prefix - ie V for VB, VC, VH etc commodore, and W for WB series.

In the 80s, the Camira was the J-platform....so if the 4 is for 1984, then could this be a super-rare Camira 308??? /smartass :D :D

My money is on a mis-stamp if it's a genuine factory stamp. Here's something to think about - think 1974 - Q was a common prefix, well known by now.
But they've just brought out H for the all-new LH series, so to a new guy, it could be plausible to think that he should put a J in there for a HJ series engine.....

Obviously if it's not a factory stamp, then who knows the where when & why for it.