Deepsea no serial number on watchcase

[Rolex116520]

Hi guys ,
i wonder why rolex has done away with putting serial numbers on the case ( between the lugs at 6 o'clock) of the sea dweller deepsea (because my v reg which i bought last week surely hasn't) . Does any earlier m reg have serials between the lugs ? They only only have it on the inside of the watch . My late z reg daytona 116520 had both inside and between the lugs the serial and so has my v reg 116520 with new bracelet (78590) given that the daytona has the inside serial far more visible then the deepsea , one can hardly read the number , especially with the thick reflecting glass , on the deepsea as it is much smaller writing . I is actually amazing how they managed to fit it on that lower inner ring . It is far less obtrusive then on other models .
But again why do away with the serial on the case ? Has it to do with corrosion risk ? I have an vintage 1601 and you cannot longer read the serial because it has been semi eaten away .