One source I have says the 424 was not imported until 1977. Here's a good clue to a GR model: Look at the profile of the receiver where it meets the head of the stock. Rather than a straight back to the receiver, there will be a rather shallow C-shaped profile. And I believe that the receiver was blued on all the GR-series guns. A GR-3, according to one source I'm looking at, should have a single trigger and extractors. That'd be my bet based on the information provided to date. If everything else matches with what I posted above, I'd say it's 99%.