This is a simple mechanical model of a photoelectric rotary encoder, just in case someone else needs to model a bracket to hold it as I am. This one uses 3x M3 screws for mounting 120 degrees apart. There are two sets of three holes; one set at 14 mm and one set at 15 mm radius from center. The features are accurate to the particular item I received. I don't know how standard this one is, but I have seen several nearly-identical photos like it on ebay listing various resolutions like 360, 400, 600 pulses per revolution. This particular one is ebay item 161297002173.

Edit Nov.15: threaded depth of M3 mounting holes is 6.2 mm
Edit Nov. 24: add "Encoder_Bracket2" with enlarged holes for use with two similar but slightly different encoder geometries