I think you will find that your rifle is not an ISU match. These were very specialized 70's built to meet the International Shooting Union requirements. They were introduced around 1970, and every one that I have ever seen was a 308. I own 2, and both are in the G1,125,XXX to G1,380,XXX range. I also have a similar one to yours and when you hold them side by side, you can see some major differences in the stock profiles. The biggest difference though is that the ISU's were made using the Palma Match or Ultra Match actions which had an externally adjustable trigger which was accessed through a small hole in the rear tang area, just to the rear of the bolt. They were all factory bedded in Bisonite in the custom shop at Winchester as well. Your rifle appears to be a '64/65 vintage based on the firing pin collar. Hope this helps.
Steve
