As noted in the remarks with the 1st image, Vaurie(1)
mentions that a significant character for R. rex
"the densely hairy elongate projection on the inner side of the front coxae at apex"
If you look between the upper-most (black) portions of the fore-leg and mid-leg, you'll see a (yellow) elongate projection covered with hair. It's better seen in the full-size
Note also that the 1st segment of the hind tarsus is obliquely truncate at its apex, and shorter than the remaining 3 tarsi together. This is the first character used in the keys of both Rivnay(2)
(1929) and Linsley & MacSwain(3)
For more complete info on this post, see 1st image in the series.