Arthur aka arolex was nice enough to give me his permission to post this excellent summary for 1680s.
With the info I have dug up this is an excellent 'guide' to 1680s. =) maverick
Based on my initial findings, and various discussions with fellow collectors, I've come to some personal conclusions.
I believe that Rolex began to distribute the 1680 Red Sub "meters first" to the general public in 1969. Which explains that all early red subs have case back date codes, no earlier then 1969. Notwithstanding the odd "unexplainable watch" or possible proto-type, all case back 1680's, that I've seen, and heard about, are stamped no early then 1969.
Now, I tend to use case back date codes, vs. serial numbers, simply because there seems to be quite a variance in serial number coding, in some case as much as a 2 year span, even on serial numbers that are quite evenly matched. Again, not sure if Rolex necessarily used a sequential serial number system, which may explain for some of the inaccuracies of the Rolex serial number coding. One only has to look at VRF's "excellent" serial number project to see the wide variance in serial number vs. year of production. Having said that, serial numbering is still useful, but not necessarily conclusive evidence.
Now back to the subject on hand, I suspect that Rolex most likely started to switch out the the meters first dial to feet first dials, around the same time that they introduced their hacking feature on their famed caliber 1570.
Now Rolex, being Rolex did not necessarily switch the dial/hacking feature in unison. This process most likely occurred over a gradual period of time, albeit short time period. In other words, there will most certainly be some form of over-lap with the possibility of non-hacking feature with feet first dials, and vice-versa.
In my crude analysis , I will limit my discussion to general terms, and thus not focus on "one-off" oddities.
So, my conclusion, based on my research/findings, and various in-depth discussions with a few Top collectors, is as follows;
1680 "meters First" had a very short life span. Most likely commencing in Q1.1969 and ending around Q. 1 or 2 .1970, coinciding with the introduction of the hacking on the 1570 caliber.
FYI, a couple of personal examples:
1. My "meters first" Red Sub is a 2.421XXXX, case back stamped 111.69, no dots on crown, original owner watch, with complete box set (Box/Org. punched papers etc.), with non-hacking movement.
2. My "feet first" Red Sub is a 2.63XXXX, case back stamped 111.70, 2nd generation Red Sub, open 6, with hacking movement, with Rolex Service papers.
"My findings are by no means conclusive, but based on personal research, and various discussions with a few top collectors, and sprinkled with a bit of logical assumptions.