A few people have made me aware that the MI_ICONS_X64.def wasn’t really up-to-date.
And you were right. I had only created this file with the icons from MapInfo Pro 12.5.2 – the initial 64 bit release.
I have now with some help from our MapInfo Pro engineering team added around 130 new icons to the file.
These were added to MapInfo Pro 15.2 and 16.0
You can find the file here on Github (it’s part of the mbLibrary repository):
https://github.com/PeterHorsbollMoller/mbLibrary/blob/master/MI_ICONS_X64.def