The prefix does not have anything to do with the build date. They start with an arbitrary number and every 100,000 guns manufactured the prefix changes by one number. The guns are not always rollmarked in sequence. When I worked at Ruger in NH they would start the build with number 500 and do the low numbers when all the bugs were worked out of the process. And some times when the spirit moved them or a subscription holder bugged them enough to get them started. If there was aproblem with a gun and it got scrapped it may or may not get re-rollmarked. I had to get special permission to get some of my serial numbered guns re-rollmarked that had been scrapped so they were way out of sequence with the rest of the low numbers. There is no set pattern as to they way they are built. Now that they have stopped the serial number subscription program that may all change, no telling. The charts that are on the Ruger website for production dates are not always accurate but are usually pretty close.