Rolex 1680 dial/serial number combinations

[Gina Marie]

I have a mark iv red 1680 serial number 2.38mm purchased in the US. I have read the red sub bible written by Mark whereby he offers that the mark iv should start at 2.45 million. I have asked other folks on the forum and been told that 2.45 is about right. How much variation is acceptable? Some claim that the mark iv in my watch must be a service dial. However my watch was sold in 1974.....if it required a service dial why would they use a mark iv and not something much later produced like a mark v or even the rare red service Swiss dial?

I have reviewed the rolex serial number project whereby the first ff red was at 2.35mm. I have seen lots of posts here by members with mark iv dials in 2.36mm watches. So could my dial be original to my watch? Could the fact that Rolex was trying to cater to a growing US market lead them to use the ff earlier than anticipated in the US? Much like Porsche....there never seems to be a correct answer with Rolex. Thoughts from those that know much more?