I am pretty sure I have seen Mustangs use the "clock" lens with a small black cap filling the hole.
+1
Did a deep dive into my documentation and found the following
Could not find an unrestored example without the same lens as used on Mustangs ordered without the clock option except for one example. Same lens with a small black plug as shown below
Each line represents Mustangs from different plants - have many more. Dearborn- NJ - San Jose from top to bottom
Just to give an idea of the the production periods and plants the data points were found I offer the following. These are unrestored Mustangs found with out clock but with the same lens. Only included examples were the lower edge of the lens was visible
8F1342xx 8F1385xx
8F1582xx 8F1709xx
8F1728xx 8F1848xx
8F1816xx 8F2203xx
8T1063xx 8T1150xx
8T1207xx 8T1454xx
8T1520xx 8T1730xx
8T2029xx 8T1817xx
8T1943xx 8T2041xx
8T2050xx
8R1303xx 8R1324xx
8R1362xx 8R1386xx
8R1393xx 8R1460xx
8R1533xx 8R1593xx
8R1603xx 8R1671xx
8R1673xx 8R1682xx
8R1769xx
And one Mexican assembled 68 Mustang
Now the one examples I found is shown below and the VIN places the car 8R03J1342xx right in the middle of a number of examples with the clock lens shown in the list above making it possibly suspect that it might have been replaced. Car was not a completely untouched car. It had been repainted (not disassembled) and the engine compartment and engine had been repainted but since it was the only one with the lens I felt it should be at least mentioned in the discussion
Did look somewhat quickly through 67s and so far the practice of using the clock lens on cars ordered without the clock was found on those cars also