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I recently picked up a custom chopper and was told the engine is from a 1971 Sportster XLCH. It's definitely an Iron Head with right side drive and shift, but the serial number on the block doesn't make sense. It is * 6A10419H1. Everything I read says a 6A is a SS-350 Sprint, although the H1 seems to match the 1971 year I was told the engine was. I just don't see how the "6" could be anything but a "6" (see picture). Any ideas?