New Old 1601 Questions

[TomsRLX]

Hi, I got an old Rolex from my grandfather and now try to get information about it. I was at the Rolex shop in Berlin, where they cleaned and checked the movement and if the watch is waterproof. They said everything is perfect.

The dial says Datejust, but it has a case diameter of 34mm without cone and a bracelet width of 20 mm at the case connection. This is irritating me, since the "datejust" should have a diameter of 36 mm with 20 mm bracelet and the "date" should have a case diameter of 34 mm with 19 mm bracelet.

I'm also want to know, what the number in pensil written number in the top right corner means, (1601/27539). A Sticker on the certificate says 1601, what is the reference number as far as I know. And the 14/2 76 is probably the purchase date I guess.

I have posted some pictures, if anyone can give me some information about the watch I would be very glad, since I'm very fascinated by the watch.