Help..question regarding my Sub dial

[bjones]

for the past 20 years, one of the Rolex watches in my small collection has been, to the best of my knowledge without pulling the serial number, a mid 70's Submariner. For all this time, until recently I never noticed it lacks the traditional "swiss made" or "swiss-T" etc. markings at the bottom of the dial. It has the crown guard and acrylic crystal and I know it is authentic since I've had it serviced a few times with a certified dealer.

Can anyone shed some light on this? I have looked at a lot of Rolex reference material but unable to find anything nor any pictures of similar Subs with the same (lack of) markings.