The description says:
The watch has been at Rolex for service before the mid of the 80's when it received an exchange case with the old serial number of the watch. How do we know that? Rolex started to use the exchange cases with 4,4 million serial around 85 when registered design was also used on new watches. This case was used before and so you can be sure that the watch was born as a real Red Sub!
Rolex case engravings went like this:
'REGISTERED DESIGN' and ref. # until 1980
<reference number only> 80-82
'ORIG. ROLEX DESIGN' and ref. # post 1982, s/n 7 million and up
So the description matches the known history of engravings. This 1680 is from 1974 and had a 3.7mil s/n. It was serviced somewhere between 1982 (first appearance of "ORIG. ROLEX DESIGN") and 1985 (Rolex starts providing exchange cases with 4.4mil s/n) and therefore RSC was able to put the original s/n on it, as well as "1680", in the different font.
Obviously a vintage piece with a service case is far less valuable than one with original case, and if you look at the other red 1974 1680s on chrono24 you'll see that this one's priced accordingly.
