Thanks for the replies and comments. I finally managed to compare with a new RD-M786 derailleur, and it's a medium size.
I think this is a Shimano XT 10 RD-M786 rear derailleur
- but is it a medium cage GS
- or a long cage SGS?
I took some measurements
- The cage plate is about 12cm to 14cm (4.72in to 5.51in) long
- The pulley wheels have diameter 4.0cm to 4.5cm (1.57in to 1.77in)
- There is enough space between the 2 pulleys to fit another pulley and some extra room left.
- The distance between the pulley wheel centers is about 10cm (3.94in)
- The ruler in the pictures is 15cm (5.9in)
- The bicycle wheel is 26in
According to this question How to calculate the capacity of a rear derailleur it should be a long (although my max cog exceeds 34)
- max chainring difference = 40 - 22 = 18
- max max cassette cog difference = 36 - 11 = 25
- total capacity = 43