By open design do you mean that the main components are accessible for service and maintenance?
or
Do you mean that it is an open design that had been openly imitated off the undisputed original - the Judson TNT.
There are several lower priced basic slides ins that look like the genuine TNT and the TNT itself can be called a lower priced slide in as well but here is the difference and it should matter if a person is to concerned about issues beyond initial purchase price.
Is the unit made of stainless steel or iron or aluminium?
The Judson difference: it's stainless which doesn't fatigue, crack, or corrode like aluminium and iron have proven themselves to. Also aluminium is very prone to damage from vibration.
What type of drive system does it have?
First of all Judson through their years of experience and testing do not believe that direct drives are in the interests of truck mount slide in owners for several reasons. The first reason is even if the units correctly balanced and coupled leaving the factory, subsequent service most often have been found to not be correctly rebalanced and aligned and that ends up shaking the crap out of the coupling which becomes a premature point of failure and source of annoying vibration.
The Judsons also do not believe in using temporary belts whose appropriate application is for emergency temporary use only. Also the Judson’s do no use cogged belts because they require absolute alignment and tension or else they will prematurely fail. Also cogged belts are harder to source than standard 3V belts found at most automotive stores. The unit can also if needed be retrofitted with 4L automotive belts.
There are more design and component differences but here are a few other differences.
A Judson unit is American made as they can get (the Cat pump is Japanese)
The Judson TNT is proven and well refined design.
Nobody has ever had a bad deal from the Judsons.
Nobody has more experience with building and service diagnosis that the Judson’s.
The Judson’s are quietly doing some major design and prototyping work for future TM releases and not looking over their shoulders to try and imitate some else.
Here is as open as you can get.
I am very happy with my TNT and I can run as hot as I want on highflow with out loosing any heat. It runs cheap and its easy to work on.
I'm pretty sure you could buy a TNT with a #4 Little Giant and a Greenhorn wand with 2.5 hose on an electric and get change back on 20 grand. What more could you want?