Those Hydro gear numbers will tell you 'what's inside' as well as torque, RPM, MPH (fwd/rev) and weight capacity. Just head to H-G website and 'search'.... you'll be able to get complete service manuals, parts list, etc.yes, it may take a few minutes, but well worth the time. Also get the 'how-to-purge' bulletin and any 'tech bulletins' that might be applicable. Knowledge is power !
Now as to why they're different HSTs for the 'same' TG1860g units, it could be one had the original HST replaced with what was on the nonKubota repair guys shelf, or ordered an 'upgraded' version of the HST( physically fits, but 'better' ). I don't KNOW the 'history' of the units,just passing on what 5 decades of 'small engine repairs'. Someone,somewhere must KNOW which H-G HST is the official Kubota replacement HST.
Right now it's a coin toss.....unless BOTH have been swapped in !!!