Hi Friedrice, you were quite the hawk-eyed there
Yup, my 1601 had the closed "6" in 26. From what I read vide Rolex forums and other watch forums, there appeared much ambiguity over the vintage date wheels which could be summarised as per the following :
1) Open 6s and 9s for all such digits;
2) Open 6s and 9s for all such digits except close 26;
3) Close 6s and 9s for all such digits.
I can't recall if the other 6s and 9s were open and looked through the pics in my blog but unfortunately had not come across those dates. Flipped the watch circa half a year back, but would be interesting if it was the rarer version that you mentioned. Some variations in the wheel date as lifted from the Net :