Look on the underside of the barrel ahead of the forearm for an "Imported by Stoeger" stamp. I'm guessing this serial number range makes it a post-Garcia, i.e. Stoeger. If imported by Garcia it will be so stamped. Yes, it is an L61R. What Sako did when they shifted serial numbers to the 500,000 range in 1974 is change the stamp from L61R to A-III. Why? Why did Sako do anything? You have already provided the answer.
I've also learned that Sako wasn't too keen on serial numbers being sequential or well documented