I find it intriguing and awesome to see what a transitional year 1970 was for the Speedmaster.
Another question... Do the serial numbers follow each version in groups, or are they mixed up? For instance, do all of the DON version serial numbers predate the DNN versions, which then all predate as a group the 220 error bezels, which then predate the straight writing versions? Or are they muddled up such that each version can be found legitimately in a serial number range of other versions?