Solo version = new is that a 19 inch version?
Just over 19" so technically not rackmountable, however we had it sitting on a rack tray on top of our SKB Mighty GigRig (no lid) and it fits comfortably so I can totally see this being used on top of AV racks.
Do you have a Picture ?
https://www.allen-heath.com/media/Weights-Dims-Avantis-Solo.pdf
The solo version is 21.5" wide and 24.7" deep. Measure you cart/rack.