If you read what the Rambam says next to the picture (its in the Pirush HaMishnayos in Menachos, Perek Gimmel), he says that he is specifically drawing this picture only to show there the gevios and prachim were on the menorah (and not to imply that the branches were diagonal; it’s just simpler to draw, print (remember, all printing back then was done with what were essentially woodcuts), and see where they were like that).
(It still could be that the picture was accurate in that regard, but there is no real reason to say that.)
As far as I can tell, every single mosaic, carving, and picture of the menorah made over 500 years ago is curved, and, in addition, nowhere in Bavli or Yerushalmi, Mechilta, Sifra, or Sifri, Medresh Rabba or Tanchuma, Breisa Dassiyas Hamishcan (and every other Mesechta of Breisos that I’ve seen) does it ever say that the branches of the Menorah were straight (although several of the less meyuchasdika breisos imply that they were curved).
I grew up hearing that it was a machlokes between the Rambam and the other Rishonim, but I’ve never seen a single Rishon (including the Rambam, although I haven’t ‘learned all of the Pirush Hamishmayos yet, nor done such a good job on the yad…) say it was curved, so I would love to know where it comes from.
Please, if anyone knows of a mekor, please tell me of one (and while if any of the Rebbes actually said it in a shiur etc. it would help, but I want a mekor from the Rishonim or earlier, if at all possible).
(Oh, and if anyone can tell me how to use hebrew (and apostrophes – they also aren’t working right) in here so that I can post the lashon of the Rambam, I’d appreciate it (1st time poster…) Thanks.)