318 race engine 4104230-318 any info. on these engines?

I just got one of these engines today and I was confused by it coming up as a "race" block. They do havr flat top pistons in them and I'm not sure if mine was ever rebuilt but it has .010 bearings but no numbers on the rods so I assume their factory bearings leading me to believe their factory flat tops. Also mine doesn't have an "x or R" stamped into it anywhere as far as I can see. It has two bolt mains. However I have a 440 with r6 stamped on the front. This may be the wrong place to ask since this is about 318's but you all seem to know a lot about block stampings. So my question is does the r6 mean anything? I know they had r3's for racing but idk about an r6? Greatly appreciate any info on the 440