JCP,
I just removed the buttstocks of four number 5 Rolling Blocks I had access to, and they all carried the "batch" numbers on the side of the upper and lower tangs. One had a "379" number, and the others had 4-digit numbers. All carried the three-line Remington tang markings with the last patent date of March 16, 1874. This supports your idea of these being early production Remington actions because late blackpowder actions also use these tang markings.
I am still not convinced that the receiver/trigger guard numbers are serial production numbers. I believe it was the late Frank DeHaas ("Mr. Single Shot") that wrote that these were receiver "batch" numbers, and did not denote serialized production.
Does anyone have access to some Remington documentation that can verify this? I've waited for two years to find a copy of the newest RRB reference book, but no on has it!