That's a good question.
The 1680 is such an iconic watch and very sought after by vintage watch collectors, It really bugs me that the rep makers just haven't come up with an accurate reproduction yet! It's probably one of the most difficult and expensive reps to mod , and rarely does it look even remotely accurate.
I have access to a lathe and have tried to shave the dial seat down with mixed results.
The dial opening is the trickiest part to get right...it has a slight angle which really takes some skill to get right.
I also have searched for local machine workshops and engineering workshops locally to me, and drawn a blank.
I am firmly of the opinion now, that after all the money spent and wasted trying to machine these cartel cases into taking gen spec dials, that you are financially better off saving for a yuki case set or Vietnam case set that is able to take a gen spec dial without machining the case , and just working on the lugs and crown guards,just my opinion!
Ive always hankered after a good 1680 and when money permits, I intend to invest in one of the more expensive case sets; I wouldn't hesitate to say that it is an investment as you will always make your money back on an accurate 1680 build .
Thats my two cents worth anyway!